.Net: Removed experimental flags for stable items (#13013)

### Motivation and Context

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Resolves: #12982

Removed experimental flags in following items:
1. `ChatClientExtensions`
2. `ChatClientBuilderExtensions`
3. `AIFunctionExtensions`
4. `ChatCompletionServiceExtensions`
5. `AuthorName` property in `ChatMessageContent` and
`StreamingChatMessageContent` models.
6. `KernelFunctionExtensions`
7. `KernelPlugin`

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Author: dmytrostruk

dotnet/src/SemanticKernel.Abstractions/AI/ChatClient/ChatClientExtensions.cs

@@ -2,7 +2,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.AI;
@@ -88,7 +87,6 @@ internal static IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
     /// The <see cref="IChatCompletionService"/>. If <paramref name="client"/> is an <see cref="IChatCompletionService"/>, <paramref name="client"/> will
     /// be returned. Otherwise, a new <see cref="IChatCompletionService"/> will be created that wraps <paramref name="client"/>.
     /// </returns>
-    [Experimental("SKEXP0001")]
     public static IChatCompletionService AsChatCompletionService(this IChatClient client, IServiceProvider? serviceProvider = null)
     {
         Verify.NotNull(client);
@@ -101,7 +99,6 @@ public static IChatCompletionService AsChatCompletionService(this IChatClient cl
     /// <summary>
     /// Get the model identifier for the specified <see cref="IChatClient"/>.
     /// </summary>
-    [Experimental("SKEXP0001")]
     public static string? GetModelId(this IChatClient client)
     {
         Verify.NotNull(client);

dotnet/src/SemanticKernel.Abstractions/AI/ChatClient/KernelFunctionInvokingChatClientBuilderExtensions.cs

@@ -1,15 +1,13 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
-using System.Diagnostics.CodeAnalysis;
 using Microsoft.Extensions.AI;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
 
 namespace Microsoft.SemanticKernel;
 
 /// <summary>Provides extensions for configuring <see cref="KernelFunctionInvokingChatClient"/> instances.</summary>
-[Experimental("SKEXP0001")]
 public static class KernelFunctionInvokingChatClientBuilderExtensions
 {
     /// <summary>

dotnet/src/SemanticKernel.Abstractions/AI/ChatCompletion/AIFunctionExtensions.cs

@@ -1,6 +1,5 @@
 // Copyright (c) Microsoft. All rights reserved.
 
-using System.Diagnostics.CodeAnalysis;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.AI;
@@ -18,7 +17,6 @@ public static class AIFunctionExtensions
     /// </summary>
     /// <param name="aiFunction">The AI function to convert.</param>
     /// <returns>The converted <see cref="KernelFunction"/>.</returns>
-    [Experimental("SKEXP0001")]
     public static KernelFunction AsKernelFunction(this AIFunction aiFunction)
     {
         Verify.NotNull(aiFunction);
@@ -35,7 +33,6 @@ public static KernelFunction AsKernelFunction(this AIFunction aiFunction)
     /// <param name="functionArguments">Contains the arguments required for the AI function execution.</param>
     /// <param name="cancellationToken">Allows for the operation to be canceled if needed.</param>
     /// <returns>The result of the function execution.</returns>
-    [Experimental("SKEXP0001")]
     public static ValueTask<object?> InvokeAsync(this AIFunction aiFunction, Kernel kernel, AIFunctionArguments? functionArguments = null, CancellationToken cancellationToken = default)
     {
         Verify.NotNull(aiFunction);

dotnet/src/SemanticKernel.Abstractions/AI/ChatCompletion/ChatCompletionServiceExtensions.cs

@@ -2,7 +2,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -119,7 +118,6 @@ public static IAsyncEnumerable<StreamingChatMessageContent> GetStreamingChatMess
     /// The <see cref="IChatClient"/>. If the <paramref name="service"/> is an <see cref="IChatClient"/>, the <paramref name="service"/>
     /// will be returned. Otherwise, a new <see cref="IChatClient"/> will be created that wraps <paramref name="service"/>.
     /// </returns>
-    [Experimental("SKEXP0001")]
     public static IChatClient AsChatClient(this IChatCompletionService service)
     {
         Verify.NotNull(service);

dotnet/src/SemanticKernel.Abstractions/Contents/ChatMessageContent.cs

@@ -17,7 +17,6 @@ public class ChatMessageContent : KernelContent
     /// <summary>
     /// Name of the author of the message
     /// </summary>
-    [Experimental("SKEXP0001")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
     public string? AuthorName
     {

dotnet/src/SemanticKernel.Abstractions/Contents/StreamingChatMessageContent.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Text;
 using System.Text.Json.Serialization;
@@ -62,7 +61,6 @@ public StreamingKernelContentItemCollection Items
     /// Name of the author of the message
     /// </summary>
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-    [Experimental("SKEXP0001")]
     public string? AuthorName
     {
         get => this._authorName;

dotnet/src/SemanticKernel.Abstractions/Functions/KernelFunctionExtensions.cs

@@ -1,6 +1,5 @@
 // Copyright (c) Microsoft. All rights reserved.
 
-using System.Diagnostics.CodeAnalysis;
 using Microsoft.Extensions.AI;
 
 namespace Microsoft.SemanticKernel;
@@ -17,7 +16,6 @@ public static class KernelFunctionExtensions
     /// <param name="kernelFunction">The <see cref="KernelFunction"/> to clone with a default <see cref="Kernel"/>.</param>
     /// <param name="kernel">The <see cref="Kernel"/> to use as the default option.</param>
     /// <param name="pluginName">Optional plugin name to use for the new kernel cloned function.</param>
-    [Experimental("SKEXP0001")]
     public static KernelFunction WithKernel(this KernelFunction kernelFunction, Kernel? kernel = null, string? pluginName = null)
     {
         var clone = kernelFunction.Clone(pluginName ?? kernelFunction.PluginName);

dotnet/src/SemanticKernel.Abstractions/Functions/KernelPlugin.cs

@@ -99,7 +99,6 @@ public IList<KernelFunctionMetadata> GetFunctionsMetadata()
     /// </remarks>
     /// <param name="kernel">The default <see cref="Kernel"/> to be used when the <see cref="AIFunction"/> is invoked.</param>
     /// <returns>An enumerable clone of <see cref="AIFunction"/> instances, for each <see cref="KernelFunction"/> in this plugin.</returns>
-    [Experimental("SKEXP0001")]
     public IEnumerable<AIFunction> AsAIFunctions(Kernel? kernel = null)
     {
         foreach (KernelFunction function in this)