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