Bump kotlin-gradle-plugin from 1.6.0 to 1.6.21

Bumps [kotlin-gradle-plugin](https://github.com/JetBrains/kotlin) from 1.6.0 to 1.6.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v1.6.21/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/compare/v1.6.0...v1.6.21)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

dependencies.gradle

@@ -9,7 +9,7 @@ ext.versions = [
 
 def gradle = "7.0.4"
 // Ref: https://kotlinlang.org/releases.html
-def kotlin = "1.6.0"
+def kotlin = "1.6.21"
 def kotlinCoroutines = "1.6.0"
 def dagger = "2.40.5"
 def retrofit = "2.9.0"