Add release schedule to the 'Upgrading Flutter' page

[itsjustkevin]

Page URL

https://docs.flutter.dev/release/upgrade/

Page source

https://github.com/flutter/website/tree/main/src/release/upgrade.md

Describe the problem

The Flutter website provides little information on the release schedule.

Expected fix

Dart has a section on each channel that describes the release schedule.

I expect there to be descriptions for each channel stating the release frequency.

Additional context

Work is already in progress for this effort, channel descriptions were added to the Flutter tool in flutter/flutter#126936.

[mit-mit]

@itsjustkevin I think we should also add this to https://docs.flutter.dev/release/archive. I say that, as this is what we link to from the Getting Started pages: "For other release channels, and older builds, check out the SDK archive."