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/components/composability_example.dart

@@ -35,11 +35,11 @@ class ParentSquare extends RectangleComponent with HasGameReference {
     ..style = PaintingStyle.stroke;
 
   ParentSquare(Vector2 position, Vector2 size)
-      : super(
-          position: position,
-          size: size,
-          paint: defaultPaint,
-        );
+    : super(
+        position: position,
+        size: size,
+        paint: defaultPaint,
+      );
 
   @override
   Future<void> onLoad() async {