chore: Update usages of Dart UI Color to not use deprecated fields and methods [flame_3d] (#3297)

Update usages of Dart UI Color to not use deprecated fields and methods.
On latest main of Flutter these will cause lint errors, [as can be seen
here](https://github.com/flame-engine/flame/actions/runs/10814047130/job/29999530293?pr=3282).

Note: I will update the `flame_3d`-specific violations on a followup.

examples/lib/stories/input/overlapping_tap_callbacks_example.dart

@@ -30,7 +30,7 @@ class TapCallbacksSquare extends RectangleComponent with TapCallbacks {
           position: position ?? Vector2.all(100),
           size: Vector2.all(100),
           paint: continuePropagation
-              ? (Paint()..color = Colors.green.withOpacity(0.9))
+              ? (Paint()..color = Colors.green.withValues(alpha: 0.9))
               : PaintExtension.random(withAlpha: 0.9, base: 100),
         );