Bump org.jetbrains.kotlin:kotlin-bom from 1.9.20 to 1.9.21

Bumps [org.jetbrains.kotlin:kotlin-bom](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-bom
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

gradle/libs.versions.toml

@@ -61,7 +61,7 @@ jsoup = "1.16.2"
 junit-testng = "1.0.4"
 junit4 = "4.13.2"
 junit5 = "5.10.1"
-kotlin = "1.9.20"
+kotlin = "1.9.21"
 lincheck = "2.18.1"
 mockito = "5.7.0"
 nexus-publish = "2.0.0-rc-1"