The default value of `GetDefaultCollationForUTF8MB4` is not righ `StaticExprContext`

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)


### 2. What did you expect to see? (Required)

The default value should be `utf8mb4_bin`

### 3. What did you see instead (Required)

It is `default_collation_for_utf8mb4`

### 4. What is your TiDB version? (Required)

master

