chore(deps): bump kotlin_version in /tutorials/kotlin/Polygons (#470)

Bumps `kotlin_version` from 1.4.30 to 1.4.31.

Updates `kotlin-gradle-plugin` from 1.4.30 to 1.4.31
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v1.4.31/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/compare/v1.4.30...v1.4.31)

Updates `kotlin-stdlib-jdk7` from 1.4.30 to 1.4.31
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v1.4.31/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/compare/v1.4.30...v1.4.31)

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

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

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 = '1.4.30'
+    ext.kotlin_version = '1.4.31'
     repositories {
         jcenter()
         google()