Migrate Plugin to Built-in Kotlin

I am using amplitude_flutter in my Flutter app.

Starting with Android Gradle Plugin (AGP) 9.0, support for applying the Kotlin Gradle Plugin (KGP) has been removed. Because this plugin applies KGP, it causes a compilation error that prevents my app from building. Here is an example of the error [Issue #181383](https://github.com/flutter/flutter/issues/181383).

Flutter has temporarily added support to allow KGP while apps and plugins migrate to AGP 9.0+, but this support will be removed in a future version of Flutter.

Please migrate this plugin to use built-in Kotlin to ensure your plugin users can successfully build their apps in future versions of Flutter.

Here is the Flutter [migration guide for plugin authors](https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-plugin-authors).