feat(firebase_core): migrate to built-in Kotlin support for AGP 9+

[JisoDev]

Description

This PR migrates firebase_core to Built-in Kotlin in accordance with Flutter's Built-in Kotlin migration guide for plugin authors.

Starting in Android Gradle Plugin (AGP) 9.0+, manual application of the Kotlin Gradle Plugin (apply plugin: 'kotlin-android') causes build failures or warnings in Flutter 3.47+.

Related Issues

Related to flutter/flutter#180686 and FlutterFire #17987.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• All existing and new tests are passing.
• I read and followed the [Flutter Style Guide].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

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[SelaseKay]

Hi @JisoDev, thanks for the PR. There's already an existing PR for this: #18355.