Migrate objectbox_flutter_libs to built-in Kotlin

I am using objectbox_flutter_libs 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: 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.

Evidence from the current published plugin version:

• objectbox_flutter_libs latest on pub.dev is 5.3.2.
• The package still includes android/build.gradle.
• That file still declares/applies KGP explicitly:
  • classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
  • apply plugin: 'kotlin-android'

Flutter warning shown during build:

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

Relevant Flutter docs:

• App developer guide: https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-app-developers
• Plugin author guide: https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-plugin-authors

Thanks.