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/system/resize_example.dart

@@ -5,9 +5,9 @@ import 'package:flame/game.dart';
 
 class ResizingRectangle extends RectangleComponent {
   ResizingRectangle()
-      : super(
-          paint: Paint()..color = const Color(0xFFFE4813),
-        );
+    : super(
+        paint: Paint()..color = const Color(0xFFFE4813),
+      );
 
   @override
   void onGameResize(Vector2 size) {