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.

packages/flame_oxygen/example/lib/system/kawabunga_system.dart

@@ -16,7 +16,9 @@ class KawabungaSystem extends BaseSystem with UpdateSystem {
     final textComponent = entity.get<TextComponent>()!;
     final textRenderer = TextPaint(
       style: textComponent.style.copyWith(
-        color: textComponent.style.color!.withOpacity(1 - timer.percentage),
+        color: textComponent.style.color!.withValues(
+          alpha: 1 - timer.percentage,
+        ),
       ),
     );