chore(deps): Bump kotlin_version from 1.9.20 to 1.9.21

Bumps `kotlin_version` from 1.9.20 to 1.9.21.

Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` 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)

Updates `org.jetbrains.kotlin:kotlin-stdlib-jdk8` 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
- 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

@@ -28,7 +28,7 @@ buildscript {
     ext.maoni_group = 'org.rm3l'
     ext.maoni_version = '10.0.0'
 
-    ext.kotlin_version = '1.9.20'
+    ext.kotlin_version = '1.9.21'
     ext.anko_version = '0.10.8'
 
     ext.aboutlibraries_version = '8.9.4'