Bump kotlin_version from 1.8.20 to 1.8.21

Bumps `kotlin_version` from 1.8.20 to 1.8.21.

Updates `org.jetbrains.kotlin.android` 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-stdlib-jdk8` 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-reflect` 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.android
  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
- dependency-name: org.jetbrains.kotlin:kotlin-reflect
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

build.gradle

@@ -4,7 +4,7 @@ buildscript {
     ext{
         compile_sdk_version = 33
         build_tool_version = '33.0.1'
-        kotlin_version = '1.8.20'
+        kotlin_version = '1.8.21'
         agp_version = '8.0.0'
         exoplayer_version = '2.18.6'
         splitties_version = '3.0.0'