chore: change all default branches to main

[bshaffer]

We should see about having Release Please skip triggering a release for all of these updates.

• 1. Merge the gh-pages PR (this must be done first because the next step will kick off docs generation)
• 2. Merge this PR
• 3. Switch the default branch of this repo and all sub-repos to main
• 4. Merge release-please PR at our leisure (as the old URLs will redirect)

[dwsupplee]

We'll also need to touch docs/manifest.json to update master->main for the docs site to work properly once everything propogates

[bshaffer]

We'll also need to touch docs/manifest.json to update master->main for the docs site to work properly once everything propogates

Done in 324f3b0

Also, added #5123 (to be merged first) to address docs site