Rename master branch to main

There's a recent movement to replace "master" with a word that is descriptive while being absent the racist history of master & slave. This [isn't a new idea](https://www.vice.com/en_us/article/8x7akv/masterslave-terminology-was-removed-from-python-programming-language), however it's gaining momentum on Twitter given the groundswell of action in the United States and globally after the police killing of George Floyd in Minneapolis two weeks ago. I stand with the protestors. 🖤

The language we use matters. Changing this for the Shields-managed repos is a minor inconvenience for us, and mostly a one-time thing, and seems well worth it.

I'd propose we rename the branch in Shields to `main`, update the open PRs to target that, and identify whatever else we have to update, like the deploy bot, deploy scripts, badges, etc.

There's a tool here we could use for the PRs: https://github.com/ethomson/retarget_prs

(I think we should also review the places where we default to master in the badge code, and think about how to handle that, however let's discuss that in #4755 and open new issues as necessary.)

No specific files or tests are named. Start by inventorying the deploy bot, deploy scripts, badges, open PR targets, and other references to the branch name, using retarget_prs for PRs; done means the branch and all required integrations consistently use main.

Written by the indexing model from the issue text.