Bump org.jetbrains.kotlin:kotlin-gradle-plugin-api from 1.9.20 to 1.9.21

Bumps [org.jetbrains.kotlin:kotlin-gradle-plugin-api](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-api
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

gradle-plugin/build.gradle.kts

@@ -19,7 +19,7 @@ allprojects {
     }
 }
 dependencies {
-    implementation("org.jetbrains.kotlin:kotlin-gradle-plugin-api:1.9.20")
+    implementation("org.jetbrains.kotlin:kotlin-gradle-plugin-api:1.9.21")
 }
 
 gradlePlugin {