Suggest Migration to Declarative Plugin DSL in Android Gradle Build

## Feature Suggestion

### Feature Description

I would like to suggest an update for the Android Gradle build configuration within the Firebase Quickstart Flutter project to utilize Gradle's declarative Plugin DSL, moving away from the legacy imperative apply script method. This suggestion aligns with the best practices recommended by Flutter from version 3.16 onwards and aims to enhance build configuration maintainability and facilitate easier future enhancements.

### Why Is This Change Important?

1. **Adherence to Best Practices**: According to Flutter's [official documentation](https://docs.flutter.dev/release/breaking-changes/flutter-gradle-plugin-apply), the recommended approach for applying Gradle plugins in Flutter projects is now through the declarative Plugin DSL. Adopting this method would ensure the project remains up-to-date with Flutter's best practices.
2. **Improved Maintainability**: By utilizing the Plugin DSL, the project's Gradle plugin management becomes cleaner and more straightforward, improving the overall maintainability of the build configuration.
3. **Future-Proofing**: Preparing the project for potential future updates that could leverage new Gradle features, possibly including the adoption of Kotlin for Gradle scripts.

### Proposed Changes

The update would involve modifications to the `android/settings.gradle` and `android/app/build.gradle` files to define and apply plugins using the declarative syntax. The key adjustments include:

- Implementing the `pluginManagement` and `plugins {}` blocks in `settings.gradle` for managing plugin versions and applications.
- Eliminating the legacy `buildscript` blocks and replacing imperative `apply plugin:` commands with the declarative `plugins {}` block in `app/build.gradle`.

リンクされている Flutter 移行ドキュメントを読み、その後 android/settings.gradle と android/app/build.gradle を確認して、現在のレガシーなプラグイン適用と提案されている宣言的な構造を比較してください。これらのファイルで plugin management ブロックと plugins ブロックが従来の buildscript と apply plugin の使用に置き換われば、作業は完了です。

索引モデルが issue の本文から書いたものです。