chore: migrate default branch to 'main'

[tseaver]

Toward: googleapis/google-cloud-python#10579

[plamut]

There's a reference to master in .github/workflows/python-publish.yml, but that probably refers to the gh-action-pypi-publish repo.

    steps:
        ...
      - name: Publish Source Distribution
        uses: pypa/gh-action-pypi-publish@master

However, I did find references to VERSION_WHITELIST in scripts/manylinux/build_on_centos.sh. I believe the inclusive language version of that should be VERSION_ALLOWLIST.

Since the purpose of master -> main migration is to address precisely these things, let's also address that in the same PR.

[tseaver]

@plamut 0f9207d renames that variable to PYTHON_VERSIONS, which is even more descriptive of its actual use: there is no "allow" going on in the script.

[dandhlee]

Confirmed no other occurrences of master, merging this to unblock #79.