Bump org.jetbrains.kotlin:kotlin-gradle-plugin (#330)

Bumps the kotlin-ecosystem group with 1 update: [org.jetbrains.kotlin:kotlin-gradle-plugin](https://github.com/JetBrains/kotlin).


Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` from 2.3.20 to 2.3.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.3.20...v2.3.21)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-version: 2.3.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: kotlin-ecosystem
...

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

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -1,5 +1,5 @@
 [versions]
-kotlin = "2.3.20"
+kotlin = "2.3.21"
 dokka = "2.2.0"
 korro = "0.2.1"
 bcv = "0.18.1"