Bump org.jetbrains.kotlin:kotlin-gradle-plugin-api in /devtools/gradle

Bumps [org.jetbrains.kotlin:kotlin-gradle-plugin-api](https://github.com/JetBrains/kotlin) from 2.0.0 to 2.0.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.0.21/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.0...v2.0.21)

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

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

devtools/gradle/gradle/libs.versions.toml

@@ -2,7 +2,7 @@
 plugin-publish = "1.2.1"
 
 # updating Kotlin here makes QuarkusPluginTest > shouldNotFailOnProjectDependenciesWithoutMain(Path) fail
-kotlin = "2.0.0"
+kotlin = "2.0.21"
 smallrye-config = "3.8.2"
 
 junit5 = "5.10.2"