x/build: update documentation and processes for new release timeline

[heschi]

For the 1.20 cycle we decided to freeze later, skip the beta releases, and reopen the tree for development early. We didn't document any of that because it was just an experiment. Now we've decided to make the changes permanent, so we should document the updated timeline, and fix any process issues.

One complaint during the early thaw was that it was hard to know how to do cherrypicks. We filed #57854 to ease the process, but people still found it confusing and filed backport issues. For the next cycle, we intend to allow all approvers to submit to the release branch for a while, so that the process is completely self-service.

Also, now that we're consistently hitting our target release dates, a new problem has emerged: doing minor releases and major releases on the same week is tough, especially when there are security fixes in the minor releases. We need to find some way to avoid this recurring conflict. One idea is to do the GA mid-month, though that means the release cycle will be about two weeks longer.

Items:

• Decide on major release timing
• Update https://github.com/golang/go/wiki/Go-Release-Cycle
  • Remove discussion of beta releases
  • Fix the dates of everything
  • Document the early thaw, and how to cherrypick CLs during the initial part.
• Design and implement Gerrit permission changes to enable easier cherrypicks during the early thaw.
  • Bonus points: automate the permission changes.

cc @golang/release

[gopherbot]

Change https://go.dev/cl/474675 mentions this issue: cmd/releaseschedule: create

[gopherbot]

Change https://go.dev/cl/479575 mentions this issue: cmd/releaseschedule: add dark mode aware svg css

[heschi]

Everything's done except the automation. Good enough for this cycle.