chore(deps): bump kotlin_version in /tutorials/kotlin/Polygons (#414)

Bumps `kotlin_version` from 1.4.21 to 1.4.21-2.

Updates `kotlin-gradle-plugin` from 1.4.21 to 1.4.21-2
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog-1.3.X.md)
- [Commits](JetBrains/kotlin@v1.4.21...v1.4.21-2)

Updates `kotlin-stdlib-jdk7` from 1.4.21 to 1.4.21-2
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog-1.3.X.md)
- [Commits](JetBrains/kotlin@v1.4.21...v1.4.21-2)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

tutorials/kotlin/Polygons/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 = '1.4.21'
+    ext.kotlin_version = '1.4.21-2'
     repositories {
         jcenter()
         google()