Bump kotlinToolchain from 2.3.20 to 2.3.21 (#1567)

Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` from 2.3.20 to 2.3.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.3.20...v2.3.21)

Updates `org.jetbrains.kotlin.plugin.serialization` from 2.3.20 to 2.3.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.3.20...v2.3.21)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-version: 2.3.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin.plugin.serialization
  dependency-version: 2.3.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -43,7 +43,7 @@ kotlinPoet = "1.6.0"
 kotlinStdLib = "2.2.21"
 #noinspection UnusedVersionCatalogEntry
 kotlinTarget = "2.2"
-kotlinToolchain = "2.3.20"
+kotlinToolchain = "2.3.21"
 kotlinxHtml = "0.12.0"
 # 1.9.0 is the last version compatible with Kotlin 2.2
 kotlinxSerialization = "1.9.0"