Update Kotlin and KSP to v2.1.21

Author: renovate[bot]

gradle/libs.versions.toml

@@ -13,7 +13,7 @@ jimfs = "1.3.0"
 jmh = "1.37"
 jsr305 = "3.0.2"
 junit = "4.13.2"
-kotlin = "2.0.21"
+kotlin = "2.1.21"
 kotlinpoet = "2.0.0"
 ktlint = "0.48.2"
 moshi = "1.15.2"