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/effects/opacity_effect_example.dart

@@ -29,14 +29,14 @@ class OpacityEffectExample extends FlameGame with TapDetector {
         position: Vector2(180, 230),
         size: Vector2.all(100),
       )..add(
-          OpacityEffect.fadeOut(
-            EffectController(
-              duration: 1.5,
-              reverseDuration: 1.5,
-              infinite: true,
-            ),
+        OpacityEffect.fadeOut(
+          EffectController(
+            duration: 1.5,
+            reverseDuration: 1.5,
+            infinite: true,
           ),
         ),
+      ),
     );
   }