Simplify release permissions process

[jmhbnz]

As discussed in last two release meetings this pull request simplifies our release permissions process by moving away from manual changes to branch restrictions and instead making the permissions based on membership of the release GitHub team.

Historically for every release we would:

1. Raise a k/org pr in advance to give release team elevated permissions by changing the permission levels for groups.
2. Once merged, manually edit our branch restrictions in GitHub so that the release team member running the release script could push the release commit directly to the stable release branch without a pull request.
3. After release raise a k/org pr to revoke release team elevated permissions by changing the permission levels for groups.

In future we will instead:

1. Raise a k/org pr in advance of each release to add release team members to the GitHub team
2. After release raise a k/org pr to revoke release team elevated permissions by removing members from release group.

By adding @etcd-io/release-etcd as an entity that can bypass the stable release branch restrictions and just managing the membership of that group we have a simpler, less manual way of carrying out future releases.

cc @ahrtr, @serathius, @ivanvc

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.85%. Comparing base (ef65923) to head (cf6f4fb).
Report is 2 commits behind head on main.

Additional details and impacted files

see 22 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19450      +/-   ##
==========================================
- Coverage   68.86%   68.85%   -0.02%     
==========================================
  Files         420      420              
  Lines       35753    35753              
==========================================
- Hits        24620    24616       -4     
  Misses       9707     9707              
- Partials     1426     1430       +4     

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[ivanvc]

LGTM. Thanks, @jmhbnz.

One less manual step 🎉

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, ivanvc, jmhbnz

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[ivanvc]

/retest