Build(deps): Bump kotlinVersion from 1.8.20 to 1.8.21

Bumps `kotlinVersion` from 1.8.20 to 1.8.21.

Updates `org.jetbrains.kotlin:kotlin-gradle-plugin` from 1.8.20 to 1.8.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/compare/v1.8.20...v1.8.21)

Updates `org.jetbrains.kotlin:kotlin-serialization` from 1.8.20 to 1.8.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/compare/v1.8.20...v1.8.21)

---
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-serialization
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

build.gradle

@@ -24,7 +24,7 @@
 buildscript {
 
     ext {
-        kotlinVersion = '1.8.20'
+        kotlinVersion = '1.8.21'
     }
 
     repositories {