Bump org.jetbrains.kotlin:kotlin-bom from 1.8.20 to 1.8.21 (#928)

Bumps [org.jetbrains.kotlin:kotlin-bom](https://github.com/JetBrains/kotlin) from 1.8.20 to 1.8.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v1.8.21/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.8.20...v1.8.21)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

gradle/libs.versions.toml

@@ -59,7 +59,7 @@ jsr330 = "1"
 junit-testng = "1.0.4"
 junit4 = "4.13.2"
 junit5 = "5.9.2"
-kotlin = "1.8.20"
+kotlin = "1.8.21"
 lincheck = "2.17"
 mockito = "5.3.1"
 nekohtml = "1.9.22"