Change channel to beta or stable

[guidezpl]

Should consider now that development has slowed, we don't have such a great need to use the master channel.

Using stable would mean devs wouldn't have to switch channels (common issue) to run the gallery.

One consideration is that flutter/flutter uses the gallery for tests, and is pinned in two places

• https://github.com/flutter/tests/blob/master/registry/flutter_gallery.test
• https://github.com/flutter/flutter/blob/master/dev/devicelab/lib/versions/gallery.dart

[wolkenschieber]

I vote for stable channel.

[parlough]

I'm going to close this due to #1072, but for those reading: Do note that the current form of the app does run on the current stable (3.16.7).

For your own apps, I highly encourage you consider developing and running your app on the beta channel. Releases on the beta channel are well tested and enable you to get fixes, performance improvements, and new features faster! It's much easier to make incremental updates as well =D

If you'd like to learn more about the channels, particularly the beta channel watch this new video on When to use the Flutter beta channel!