Bump kotlin_version from 1.6.20 to 1.6.21

Bumps `kotlin_version` from 1.6.20 to 1.6.21.

Updates `kotlin-gradle-plugin` from 1.6.20 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/v1.6.20...v1.6.21)

Updates `kotlin-stdlib` from 1.6.20 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/v1.6.20...v1.6.21)

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

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

build.gradle

@@ -1,7 +1,7 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
 
 buildscript {
-    ext.kotlin_version = '1.6.20'
+    ext.kotlin_version = '1.6.21'
     repositories {
         google()
         mavenCentral()