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/widgets/sprite_animation_widget_example.dart

@@ -21,13 +21,14 @@ Widget spriteAnimationWidgetBuilder(DashbookContext ctx) {
       anchor: Anchor.valueOf(
         ctx.listProperty('anchor', 'center', anchorOptions),
       ),
-      paint: paintList[paintChoices.indexOf(
-        ctx.listProperty(
-          'paint',
-          'none',
-          paintChoices,
-        ),
-      )],
+      paint:
+          paintList[paintChoices.indexOf(
+            ctx.listProperty(
+              'paint',
+              'none',
+              paintChoices,
+            ),
+          )],
     ),
   );
 }