Replace ButtonBar with OverflowBar (#2072)

This PR replaces `ButtonBar` with `OverflowBar` in preparation of
`ButtonBar` deprecation

related to https://github.com/flutter/flutter/issues/127955


## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

isolate_example/lib/infinite_process_page.dart

@@ -54,20 +54,24 @@ class InfiniteProcessPage extends StatelessWidget {
           Column(
             mainAxisSize: MainAxisSize.min,
             children: [
-              ButtonBar(
-                alignment: MainAxisAlignment.center,
-                children: [
-                  ElevatedButton(
-                    style: ElevatedButton.styleFrom(elevation: 8.0),
-                    onPressed: () => controller.start(),
-                    child: const Text('Start'),
-                  ),
-                  ElevatedButton(
-                    style: ElevatedButton.styleFrom(elevation: 8.0),
-                    onPressed: () => controller.terminate(),
-                    child: const Text('Terminate'),
-                  ),
-                ],
+              Padding(
+                padding: const EdgeInsets.all(8.0),
+                child: OverflowBar(
+                  spacing: 8.0,
+                  alignment: MainAxisAlignment.center,
+                  children: [
+                    ElevatedButton(
+                      style: ElevatedButton.styleFrom(elevation: 8.0),
+                      onPressed: () => controller.start(),
+                      child: const Text('Start'),
+                    ),
+                    ElevatedButton(
+                      style: ElevatedButton.styleFrom(elevation: 8.0),
+                      onPressed: () => controller.terminate(),
+                      child: const Text('Terminate'),
+                    ),
+                  ],
+                ),
               ),
               Row(
                 mainAxisAlignment: MainAxisAlignment.center,