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"
/>

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

@@ -6,9 +6,9 @@ import 'package:flutter/material.dart';
 class KawabungaSystem extends BaseSystem with UpdateSystem {
   @override
   List<Filter<Component>> get filters => [
-        Has<TextComponent>(),
-        Has<TimerComponent>(),
-      ];
+    Has<TextComponent>(),
+    Has<TimerComponent>(),
+  ];
 
   @override
   void renderEntity(Canvas canvas, Entity entity) {
@@ -16,8 +16,9 @@ class KawabungaSystem extends BaseSystem with UpdateSystem {
     final textComponent = entity.get<TextComponent>()!;
     final textRenderer = TextPaint(
       style: textComponent.style.copyWith(
-        color:
-            textComponent.style.color!.withValues(alpha: 1 - timer.percentage),
+        color: textComponent.style.color!.withValues(
+          alpha: 1 - timer.percentage,
+        ),
       ),
     );
 

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

@@ -36,11 +36,11 @@ class MoveSystem extends System with UpdateSystem, GameRef<ExampleGame> {
         velocity.setFrom(-velocity);
 
         game!.createEntity(
-          name: '${entity.name} says',
-          position: position + size / 2,
-          size: Vector2.zero(),
-          anchor: Anchor.topCenter,
-        )
+            name: '${entity.name} says',
+            position: position + size / 2,
+            size: Vector2.zero(),
+            anchor: Anchor.topCenter,
+          )
           ..add<TextComponent, TextInit>(
             TextInit(
               'Kawabunga',