INCLUSIVITY: Change default branch from `master` to `main`

[mrmundt]

In support of using more inclusive language, I recommend that efforts be taken across all of bssw-psip repositories to change the default branches from master to main.

[gonsie]

I'm happy to do this (I have admin) or give you admin to achieve the change. GitHub has an easy interface to rename, there are some caveats (from the dialog box):

Renaming this branch:

• Will unpublish current GitHub Pages site
  Your current GitHub Pages site will become unpublished. A new commit on the renamed branch will publish the GitHub Pages site again.
• Will not update your members' local environments.
  Your members will have to manually update their local environments. We'll let them know when they visit the repository, or you can share the following commands.

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a
Rename branch

[mrmundt]

@gonsie - Whichever you would prefer, thank you!