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 https://github.com/flutter/flutter/pull/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."

[parlough]

This was added in https://github.com/flutter/website/commit/c0b2480e045bb381ad49df8d766e344574dbdfc5 by @antfitch to the SDK archive overview.

If anyone would like further changes or clarifications, please feel free to open a new issue or reopen this one with more information. Thanks all!