More prow actions

[blackpiglet]

Thank you for contributing to Velero!

Please add a summary of your change

This PR introduces more k8s PROW style commands, including:

• approve: approve PR, e.g. /approve.
• assign: assign issue or PR to specified users, e.g. /assign @blackpiglet @reasonerjt.
• close: close an issue or PR, e.g. /close.
• hold: add a hold label to an issue or PR. This label blocks the auto-merging, e.g. /hold to hold the PR from auto-merging or /hold cancel to cancel the hold.
• milestone: add the issue or PR to a specified milestone, e.g. /milestone v1.14.
• retitle: modify the issue's title or PR, e.g. /retitle Introduce more PROW actions.
• remove: remove specified labels from issue and PR, e.g. /remove kind/changelog-not-required.
• reopen: reopen the issue or PR, e.g. /reopen.
• unassign: unassign the specified users from the issue or PR, e.g. /unassign @blackpiglet @reasonerjt.

Also, add some introduction to the already supported PROW commands:

• kind: add labels starting with kind/, e.g. /kind changelog-not-required release-note.
• area: add labels starting with Area/ or area/, e.g. /area CLI CSI.
• cc: request review from the specified users, e.g. /cc @blackpiglet @reasonerjt.
• uncc: remove the specified users from the reviewers, e.g. /uncc @blackpiglet @reasonerjt.

Please put those commands in the issue or the PR's comments to try them out.

Does your change fix a particular issue?

Fixes #7783

This PR is based on the ongoing PR #7776.

Please indicate you've done the following:

• [x] Accepted the DCO. Commits without the DCO will delay acceptance.
• [ ] Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• [ ] Updated the corresponding documentation in site/content/docs/main.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.64%. Comparing base (f7c0244) to head (83c082f). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7784 +/- ## ========================================== - Coverage 58.66% 58.64% -0.03% ========================================== Files 344 344 Lines 28731 28731 ========================================== - Hits 16854 16848 -6 - Misses 10448 10453 +5 - Partials 1429 1430 +1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[blackpiglet]

The lgtm command is not included in this PR intentionally. Auto-merging seems too much for now. Let's get familiar with the PROW-style workflow first.

[blackpiglet]

/cc @kaovilai

[blackpiglet]

/cc @mmorel-35 @kaovilai @qiuming-best

[kaovilai]

Don't think cc worked for me

[blackpiglet]

/cc doesn't work for most cases. It only works for the maintainers' IDs listed in the collaborator list. It doesn't even work for those in the collaborator list through an alias.