[workmanager_android] Migrate Plugin to Built-in Kotlin

[jayantur13]

Hey,

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.

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.

The body is copied from official template. Actual message from my terminal

WARNING: Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): workmanager_android
Future versions of Flutter will fail to build if your app uses plugins that apply KGP

Thanks.