Bump kotlin_version from 1.8.20 to 1.8.21 (#442)

Bumps `kotlin_version` 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/commits)

Updates `org.jetbrains.kotlin:kotlin-stdlib` 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/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
  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>
Co-authored-by: ligi <ligi@ligi.de>

Author: dependabot[bot]

build.gradle

@@ -1,6 +1,6 @@
 buildscript {
     ext {
-        kotlin_version = '1.8.20'
+        kotlin_version = '1.8.21'
 	// wait for upstream https://github.com/mockito/mockito/issues/2007
         mockito_version = '3.4.6'
         threetenbp_version = '1.6.8'