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/rendering/particles_example.dart

@@ -432,7 +432,9 @@ class ParticlesExample extends FlameGame {
             renderer: (canvas, particle) {
               final paint = randomElement(paints);
               // Override the color to dynamically update opacity
-              paint.color = paint.color.withOpacity(1 - particle.progress);
+              paint.color = paint.color.withValues(
+                alpha: 1 - particle.progress,
+              );
 
               canvas.drawCircle(
                 Offset.zero,