Part of umbrella issue to rename default branches: kubernetes/org#2222

Copy-pasted checklist from https://raw.githubusercontent.com/kubernetes/community/master/github-management/default-branch-migration.md

Changes pre-rename

Make the following changes before renaming the branch the master branch.

Note: There might be additional changes required that have not been
covered in this checklist.

Anytime

These changes are non-disruptive and can be made anytime before
renaming the branch.

• If a prowjob triggers on the master branch (branches field
  of the prowjob), add the main branch to the list
  (see [prep kubernetes/org jobs for default branch rename kubernetes/test-infra#20665] for an example).

  • searched for jobs: https://cs.k8s.io/?q=slack-infra&i=nope&files=config%2Fjobs&excludeFiles=&repos=kubernetes/test-infra
  • opened PR: jobs: prep slack-infra for default branch rename kubernetes/test-infra#21607

• If the [milestone_applier] prow config references the master branch,
  add the main branch to the config (see [Prep kubernetes/k8s.io for default branch rename kubernetes/test-infra#20675] for an example).

  • n/a - no milestone_applier setting

• If the [branch_protection] prow config references the master branch,
  add the main branch to the config.

  • n/a - no branch_protection setting

Just before rename

These changes are disruptive and should be made just before
renaming the branch.

• If a prowjob mentions the master branch in base_ref,
  update it to the main branch. For a periodic job, ensure that
  the branch is renamed between periodic job runs.

  • n/a - no jobs do this

• If a prowjob mentions master in its name, rename the job to
  to not include the branch name. [status-reconciler] should automatically
  migrate the PR status contexts to the new job name but this has not been tested yet.
  The job with the new name will also appear as a differt job in Testgrid.

  • n/a - no slack-infra jobs do this

• If a prowjob calls scripts or code in your repo that explicitly
  reference master, update all references to use main.

• If the repo has netlify configured for it, ask a member of the GitHub
  Management Team to rename the master branch to main in the netlify site config.
  It can't be controlled through the netlify config in the repo.

  • n/a - no netlify

Approval

• Once all non-disruptive tasks have been completed and disruptive tasks
  have been identified, assign the GitHub Management team ([@kubernetes/owners])
  for approval.
  • verbally asked during github management meeting

Rename the default branch

• Rename the default branch from master to main using the GitHub UI
  by following the [official instructions].
  • confirmed no prowjob retriggers when clicking the button, reported on prow: avoid retriggering presubmits on default remote branch rename kubernetes/test-infra#20829 and closed that issue

Changes post-rename

After the default branch has been renamed to main, make the following
changes.

Note: There might be additional changes required that have not been
covered in this checklist.

Prowjobs

• If a prowjob still references the master branch in the branches field,
  remove the master branch (see [Update kubernetes/org job configs post default-branch-rename kubernetes/test-infra#20669] for an example).
  • jobs: cleanup slack-infra after branch rename kubernetes/test-infra#21617

Prow config

• If the [milestone_applier] prow config references the master branch,
  remove it from the config.

  • n/a

• If the [branch_protection] prow config references the master branch,
  remove it from the config.

  • n/a

Other

• If any docs reference the master branch, update to main
  (URLs will be automatically redirected).

  • Rename kubernetes-sigs/slack-infra default branch kubernetes/community#5687

• Ensure that CI and PR tests work fine.

  • ran /test all on a sample PR
  • merged a sample PR
  • no periodics/postsubmits to test

• Trial the local development experience with a pre-rename clone.

  • did so

• Send a notice about the branch rename to your SIG's mailing list.
  Include the link to the [GitHub instructions to rename your local branch].

  • https://groups.google.com/g/kubernetes-sig-contribex/c/rRnvJh2WROc/m/acEQqPODAQAJ

/assign
/sig contributor-experience
/kind cleanup