chore: Final tweaks to prepare merging flame_3d (#3381)

Final tweaks to prepare merging flame_3d.

As can be seen [on the merge
PR](https://github.com/flame-engine/flame/pull/3377), we are almost
ready to have a nice diff to merge. This fixes a couple final issues:

* the `.4 -> 0.4` thing that I had copied from main before merging my
fix into main (this will "untouch" that file from the main diff)
* the min flutter version as set by `melos bs` on the flame_3d package
itself

examples/lib/stories/system/resize_example.dart

@@ -13,7 +13,7 @@ class ResizingRectangle extends RectangleComponent {
   void onGameResize(Vector2 size) {
     super.onGameResize(size);
 
-    this.size = size * .4;
+    this.size = size * 0.4;
   }
 }