Bump kotlin from 2.1.20 to 2.1.21

Bumps `kotlin` from 2.1.20 to 2.1.21.

Updates `org.jetbrains.kotlin.plugin.atomicfu` 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](https://github.com/JetBrains/kotlin/commits)

Updates `org.jetbrains.kotlin.jvm` 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](https://github.com/JetBrains/kotlin/commits)

Updates `org.jetbrains.kotlin.multiplatform` 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](https://github.com/JetBrains/kotlin/commits)

Updates `org.jetbrains.kotlin.plugin.serialization` 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](https://github.com/JetBrains/kotlin/commits)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin.plugin.atomicfu
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin.jvm
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin.multiplatform
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin.plugin.serialization
  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

@@ -17,7 +17,7 @@ binary-compatibility-validator = "0.17.0"
 dokka = "2.0.0"
 hildan-kotlin-publish-plugin = "1.7.0"
 hildan-github-changelog-plugin = "2.2.0"
-kotlin = "2.1.20"
+kotlin = "2.1.21"
 kotlinpoet = "2.1.0"
 kotlinx-atomicfu = "0.27.0"
 kotlinx-coroutines = "1.10.2"