Remove dev channel from Flutter builds

[khyati82]

This issue tracks all the action items required for removing dev channel from Flutter builds. The entire premise of the discussion (why and what) has been documented here for additional context.

Implementation Tasks

• Update Flutter tool to handle people currently on the dev channel to auto migrate/guide them onto another channel #94967
• Update wiki to reflect current customer promises #94969
• Update deprecation policy to account for there being no dev channel #94970
• Update recommendations for people who want to know which master build passed our tests #94971
• Update our website in places where it refers to the dev channel #94973
• Remove dev channel support from DartPad #94974. This bug is mirrored in dartpad repo to track backend work - #2111
• Remove dev channel from pre-submit tests #96428
• Remove dev channel mention on website

[Hixie]

go/flutter-dev-channel-removal

Looks like that's a Google-internal link, is there a public link?

[khyati82]

Removed it for now. Will add public link.

[timsneath]

@khyati82 Another item to add to this list is that we should remove the dev channel from presubmit tests on the website (and anywhere else). See, for example, https://github.com/flutter/website/runs/4767648310?check_suite_focus=true