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/input/hover_callbacks_example.dart

@@ -37,11 +37,11 @@ class HoverSquare extends PositionComponent with HoverCallbacks {
   static final Paint _grey = Paint()..color = const Color(0xFFA5A5A5);
 
   HoverSquare(Vector2 position)
-      : super(
-          position: position,
-          size: Vector2.all(100),
-          anchor: Anchor.center,
-        );
+    : super(
+        position: position,
+        size: Vector2.all(100),
+        anchor: Anchor.center,
+      );
 
   @override
   void render(Canvas canvas) {