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/utils/timer_component_example.dart

@@ -37,10 +37,10 @@ class RenderedTimeComponent extends TimerComponent {
   final double yOffset;
 
   RenderedTimeComponent(double period, {this.yOffset = 150})
-      : super(
-          period: period,
-          removeOnFinish: true,
-        );
+    : super(
+        period: period,
+        removeOnFinish: true,
+      );
 
   @override
   void render(Canvas canvas) {