Rename GitHub branches from master to main

[manics]

Follow-up from #299. This suggested workflow is based on jupyterhub/jupyterhub#3462 and jupyterhub/zero-to-jupyterhub-k8s#2217

1. Check that master is the default GitHub repo branch. If it's not you probably don't need to do anything 😃
2. Replace master with HEAD in all URLs pointing to a JupyterHub GitHub file or directory: https://github.com/jupyterhub/.+/(blob|tree)/master/.*
3. Optionally replace master with HEAD in GitHub URLs for non-JupyterHub repos (only do this if master is the default branch, it usually is but that's not guaranteed)
4. Update CI workflows that reference the master branch. If they mention master and main remove master to avoid problems if someone inadvertently pushes a new master branch
5. Update references to master branches in docs
6. Open a PR with using this template (edit as required):

Actions to take, in order:

- [ ] approve this PR
- [ ] rename the `master` branch to `main` in GitHub (this should automatically update all PRs)
- [ ] merge this PR
- [ ] check the publish workflow publishes the expected outputs if relevant
- [ ] If readthedocs.org is used, set main as the default branch explicitly: admin -> advanced -> default branch

`$ git grep master`
# Include the above output to help reviewers. If any mentions of `master` aren't obvious add an explanation.

Active repos with a default master branch

Obtained with https://gist.github.com/manics/6c8b88b191fae7dcf7a8a8cc5fc3cdc9

• https://github.com/jupyterhub/.github
• https://github.com/jupyterhub/autodoc-traits
• https://github.com/jupyterhub/batchspawner
• https://github.com/jupyterhub/mybinder.org-user-guide
• https://github.com/jupyterhub/binder-billing
• https://github.com/jupyterhub/binder-data
• https://github.com/jupyterhub/binderhub
• https://github.com/jupyterhub/chartpress
• https://github.com/jupyterhub/design
• https://github.com/jupyterhub/dockerspawner
• https://github.com/jupyterhub/firstuseauthenticator
• https://github.com/jupyterhub/helm-chart
• https://github.com/jupyterhub/hubshare
• https://github.com/jupyterhub/jupyter-rsession-proxy
• https://github.com/jupyterhub/jupyter-server-proxy
• https://github.com/jupyterhub/jupyterhub-deploy-docker
• https://github.com/jupyterhub/jupyterhub-deploy-hpc
• https://github.com/jupyterhub/jupyterhub-deploy-teaching
• https://github.com/jupyterhub/jupyterhub-example-kerberos
• https://github.com/jupyterhub/jupyterhub-idle-culler
• https://github.com/jupyterhub/jupyterhub-the-hard-way
• https://github.com/jupyterhub/jupyterhub-tutorial
• https://github.com/jupyterhub/kubespawner
• https://github.com/jupyterhub/ldapauthenticator
• https://github.com/jupyterhub/ltiauthenticator
• https://github.com/jupyterhub/mybinder-tools
• https://github.com/jupyterhub/mybinder.org-deploy
• https://github.com/jupyterhub/nativeauthenticator
• https://github.com/jupyterhub/nbgitpuller
• https://github.com/jupyterhub/nullauthenticator
• https://github.com/jupyterhub/oauthenticator
• https://github.com/jupyterhub/outreachy
• https://github.com/jupyterhub/pebble-helm-chart
• https://github.com/jupyterhub/repo2docker
• https://github.com/jupyterhub/repo2docker-action
• [ ] https://github.com/jupyterhub/simplespawner
• https://github.com/jupyterhub/sudospawner
• https://github.com/jupyterhub/systemdspawner
• https://github.com/jupyterhub/team-compass
• https://github.com/jupyterhub/the-littlest-jupyterhub
• https://github.com/jupyterhub/tmpauthenticator
• https://github.com/jupyterhub/traefik-proxy
• https://github.com/jupyterhub/wrapspawner
• https://github.com/jupyterhub/yarnspawner
• https://github.com/jupyterhub/jupyterhub-on-hadoop
• https://github.com/jupyterhub/kerberosauthenticator