[pigeon]: Bump org.jetbrains.kotlin:kotlin-gradle-plugin

Bumps [org.jetbrains.kotlin:kotlin-gradle-plugin](https://github.com/JetBrains/kotlin) from 1.8.20 to 1.8.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.8.20...v1.8.21)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

packages/pigeon/platform_tests/test_plugin/android/build.gradle

@@ -2,7 +2,7 @@ group 'com.example.test_plugin'
 version '1.0-SNAPSHOT'
 
 buildscript {
-    ext.kotlin_version = '1.8.20'
+    ext.kotlin_version = '1.8.21'
     repositories {
         google()
         mavenCentral()