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-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](JetBrains/kotlin@v1.4.20...v1.4.21)

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

Author: dependabot[bot]

build.gradle

@@ -1,5 +1,5 @@
 buildscript {
-    ext.kotlin_version = '1.4.20'
+    ext.kotlin_version = '1.4.21'
     repositories {
         google()
         jcenter()