Bump org.jetbrains.kotlin:kotlin-gradle-plugin from 1.9.20 to 1.9.21

Bumps [org.jetbrains.kotlin:kotlin-gradle-plugin](https://github.com/JetBrains/kotlin) from 1.9.20 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@v1.9.20...v1.9.21)

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

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

gradle/libs.versions.toml

@@ -2,7 +2,7 @@
 # build tools
 java = "17"
 jacoco = "0.8.8"
-kotlin = "1.9.20"
+kotlin = "1.9.21"
 
 [libraries]
 # build tools