Update master to main

Folks, just a heads up that we'll be swapping the primary branch from master to main. GitHub's tooling will remap any open PRs. If you're working locally and all your code has been merged, you can either reset your origin (GitHub will provide an instruction) or just do a fresh clone.

cc/ @leebyron

The issue names no files, tests, or entry points; begin by checking the repository's primary-branch setting and any documented references to master. Done means the primary branch is main and affected guidance is consistent.

Written by the indexing model from the issue text.