Bump org.jetbrains.kotlin:kotlin-gradle-plugin from 2.1.20 to 2.1.21

Bumps [org.jetbrains.kotlin:kotlin-gradle-plugin](https://github.com/JetBrains/kotlin) from 2.1.20 to 2.1.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.1.20...v2.1.21)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -4,7 +4,7 @@ openapiparser = "2025.3-SNAPSHOT"
 build-jdk = "11"
 test-jdk = "17"
 
-kotlin = "2.1.20"
+kotlin = "2.1.21"
 checker = "3.49.3"
 checkerp = "0.6.52"
 jacoco = "0.8.11"