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

@@ -4,7 +4,7 @@ compileSdk = "34"
 minSdk = "21" # was "24" in desktop
 wearMinSdk = "25"
 
-kotlin = "1.9.20" # https://kotlinlang.org/docs/releases.html#release-details
+kotlin = "1.9.21" # https://kotlinlang.org/docs/releases.html#release-details
 agp = "8.1.4" # https://developer.android.com/studio/releases/gradle-plugin
 realm = "1.13.0" # https://github.com/realm/realm-kotlin