chore: Update min Dart constraint to 3.8 (#3676)

Update min Dart constraint to 3.8, which will enable us to use the
fancier collection literals.

This requires bumping the min Flutter version as well:

<img width="1892" height="1122" alt="image"
src="https://github.com/user-attachments/assets/7c7b07fc-4d96-4987-824d-9a7133ecfb85"
/>

examples/lib/stories/structure/structure.dart

@@ -4,7 +4,9 @@ import 'package:examples/stories/structure/levels.dart';
 import 'package:flame/game.dart';
 
 void addStructureStories(Dashbook dashbook) {
-  dashbook.storiesOf('Structure').add(
+  dashbook
+      .storiesOf('Structure')
+      .add(
         'Levels',
         (_) => GameWidget(game: LevelsExample()),
         info: LevelsExample.description,