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

Updates `kotlin-stdlib-jdk7` 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](https://github.com/JetBrains/kotlin/compare/v1.4.20...v1.4.21)

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

build.gradle

@@ -8,7 +8,7 @@ import com.github.spotbugs.snom.SpotBugsTask
 // its root folder, another one for the app module in subfolder of root.
 
 buildscript {
-    ext.kotlin_version = '1.4.20'
+    ext.kotlin_version = '1.4.21'
     repositories {
         google()
         jcenter()