[blocked] release v3

[tswast]

Since there are multiple branches, this will be tricky and require admin help.

• Make pre-release(s) of v3 branch and publish to PyPI.
• Test connectors, such as pandas-gbq and sqlalchemy-bigquery with pre-release, make updates if needed.
• Cut a final release of 2.x via main branch.
• DO NOT MERGE any more PRs to main branch.
• Sync v3 with main
• Rename main to v2
• Rename v3 to main
• Set main as default branch.
• Update branch protection rules.
• Release 3.0
• [Optional?] remove v2 branch

[tswast]

We're getting awfully close to the holidays. I'm thinking we might want to wait to 2022 before cutting over to v3. In the meantime, we should make some pre-releases so that connectors like pandas-gbq and sqlalchemy-bigquery can ensure they remain compatible with both v2 and v3.

[tswast]

Pre-release "beta 1" published: https://pypi.org/project/google-cloud-bigquery/3.0.0b1/