Bump kotlin_version from 2.0.20 to 2.0.21

Bumps `kotlin_version` from 2.0.20 to 2.0.21.

Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` from 2.0.20 to 2.0.21
- [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:kotlin-stdlib-jdk8` from 2.0.20 to 2.0.21
- [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:kotlin-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin:kotlin-stdlib-jdk8
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

build.gradle

@@ -1,7 +1,7 @@
 import org.gradle.internal.jvm.Jvm
 
 buildscript {
-    ext.kotlin_version = '2.0.20'
+    ext.kotlin_version = '2.0.21'
     repositories {
         google()
         mavenCentral()