Bump kotlin from 1.9.0 to 1.9.21

Bumps `kotlin` from 1.9.0 to 1.9.21.

Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` from 1.9.0 to 1.9.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.0...v1.9.21)

Updates `org.jetbrains.kotlin:kotlin-allopen` from 1.9.0 to 1.9.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.0...v1.9.21)

Updates `org.jetbrains.kotlin:kotlin-serialization` from 1.9.0 to 1.9.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.0...v1.9.21)

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

Signed-off-by: dependabot[bot] <[email protected]>

gradle/libs.versions.toml

@@ -1,5 +1,5 @@
 [versions]
-kotlin = "1.9.0"
+kotlin = "1.9.21"
 kotlinx-serialization = "1.6.0"
 benchmark = "0.4.9"