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/rendering/particles_example.dart

@@ -327,7 +327,8 @@ class ParticlesExample extends FlameGame {
   Particle acceleratedParticles() {
     return Particle.generate(
       generator: (i) => AcceleratedParticle(
-        speed: Vector2(
+        speed:
+            Vector2(
               rnd.nextDouble() * 600 - 300,
               -rnd.nextDouble() * 600,
             ) *
@@ -432,8 +433,9 @@ class ParticlesExample extends FlameGame {
             renderer: (canvas, particle) {
               final paint = randomElement(paints);
               // Override the color to dynamically update opacity
-              paint.color =
-                  paint.color.withValues(alpha: 1 - particle.progress);
+              paint.color = paint.color.withValues(
+                alpha: 1 - particle.progress,
+              );
 
               canvas.drawCircle(
                 Offset.zero,