firebase_ui_auth: widen app_links constraint to allow ^7.0.0

`firebase_ui_auth` 3.1.0 pins `app_links: ^6.4.0`. `app_links` 7.x has been out for a while (currently 7.2.1) and includes the UIScene lifecycle adoption, so any app using `firebase_ui_auth` is held on 6.4.1 and gets this on every iOS launch:

```
Plugin AppLinksIosPlugin uses deprecated application lifecycle events. Please contact plugin maintainers and request UIScene lifecycle support. This will be required in a future version of Flutter.
```

Flutter has said these warnings will become errors in a future release, so this will eventually break builds rather than just being noise.

From the app_links changelog the 6 to 7 bump looks small on the consuming side. Would you accept a PR widening the constraint to `">=6.4.0 <8.0.0"` (or straight `^7.0.0`) if the API usage in firebase_ui_auth is compatible?

firebase_ui_auth パッケージの依存関係宣言を特定して app_links API の使用箇所を確認し、その使用方法を app_links 7.x の changelog と比較します。パッケージに既存のテストと依存関係解決のチェックを実行します。対応する制約が意図した app_links リリースを API またはビルドの失敗なしに受け入れ、報告された iOS のライフサイクル警告に対処できれば完了です。

索引モデルが issue の本文から書いたものです。