Bump kotlin_version from 1.4.20 to 1.4.21

Bumps `kotlin_version` from 1.4.20 to 1.4.21.

Updates `kotlin-gradle-plugin` from 1.4.20 to 1.4.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.4.20...v1.4.21)

Updates `kotlin-stdlib` from 1.4.20 to 1.4.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.4.20...v1.4.21)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

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 = '1.4.20'
+        kotlin_version = '1.4.21'
         lifecycle_version = "2.2.0"
         appcompat_version = "1.2.0"
         constraint_layout_version = "2.0.4"