slack-21.0: init v21 release

[timvaillancourt]

Description

Init the slack-21.0 release, based on v21.0.3

Details:

• Use https://github.com/slackhq/vitess/tree/slack-19.0 as the previous release
• Setup private repo ACLs for https://github.com/slackhq/vitess-addons
• Modify CODEOWNERS to require review from @slackhq/vitess-approvers
• Modify the "16 cores" runner to use slackhq runner: vitess-ubuntu24-16cpu-1
  • Test failed on our Ubuntu 20.x runner
  • Backported v22 PR to simplify this change:
    • Simply changing GH Actions runner vitessio/vitess#17788
    • nit: move new consts to own section vitessio/vitess#17802
• Remove CI jobs that aren't relevant to this fork, namely mysql57 testing

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes