chore: Upgrade Flutter setup action (更新 Flutter setup action)

- Upgrade `flutter-action` from `v2` to `v4` (将 `flutter-action`
  从 `v2` 升级到 `v4`)
- Use `flutter-actions/setup-flutter` for better maintenance
  (使用 `flutter-actions/setup-flutter` 以获得更好的维护)

Author: CaiJingLong

.github/workflows/release.yml

@@ -51,7 +51,7 @@ jobs:
           fetch-depth: 0
 
       - name: Setup Flutter
-        uses: subosito/flutter-action@v2
+        uses: flutter-actions/setup-flutter@v4
         with:
           channel: stable
           cache: true
@@ -103,7 +103,7 @@ jobs:
           fetch-depth: 0
 
       - name: Setup Flutter
-        uses: subosito/flutter-action@v2
+        uses: flutter-actions/setup-flutter@v4
         with:
           channel: stable
           cache: true