chore(deps): bump org.jetbrains.kotlin:kotlin-gradle-plugin

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]

build.gradle.kts

@@ -6,7 +6,7 @@ buildscript {
     dependencies {
         classpath("io.realm:realm-gradle-plugin:10.19.0")
         // required by com.mikepenz.aboutlibraries.plugin
-        classpath ("org.jetbrains.kotlin:kotlin-gradle-plugin:2.1.20")
+        classpath ("org.jetbrains.kotlin:kotlin-gradle-plugin:2.1.21")
     }
 }