Rename master branch to main

[manics]

Actions to take, in order:

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

Remaining mentions:

$ git grep -c master 
CHANGES.md:3
CONTRIBUTING.md:1
binderhub/_version.py:10
binderhub/builder.py:3
binderhub/main.py:1
binderhub/repoproviders.py:5
binderhub/tests/http-record.nbviewer.jupyter.org.json:4
binderhub/tests/http-record.www.hydroshare.org.json:1
binderhub/tests/http-record.zenodo.org.json:1
binderhub/tests/test_main.py:9
binderhub/tests/test_repoproviders.py:4
helm-chart/.gitignore:1
helm-chart/images/binderhub/requirements.in:1
versioneer.py:21

https://github.com/jupyterhub/team-compass/issues/412

[consideRatio]

Should we go for a merge and follow the outlined action plan at this point? I'd love to get this merged to avoid conflicts with future work.

[manics]

If you've got time go ahead!

[consideRatio]

@manics all checkboxes ticked without issues, thank you!!!