upgrade kotlin - 1.9.21

Bumps [org.jetbrains.kotlin:kotlin-gradle-plugin](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-gradle-plugin
  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>

build.gradle.kts

@@ -4,7 +4,7 @@ buildscript {
         // https://developer.android.com/studio/releases/gradle-plugin
         classpath("com.android.tools.build:gradle:8.1.4")
         // https://kotlinlang.org/releases.html
-        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.20")
+        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.21")
     }
 
     repositories {