Change COmanage Registry GitHub default branch name from master to main

Done by Benn:

• Browse to https://github.com/Internet2/comanage-registry
• Click on "Settings"
• Click on "Branches" on left-hand menu
• Click on the pencil icon next to the default branch name (master)
• In the box replace 'master' with 'main'
• Click "Rename branch"

Done by everyone else (and Benn) with local checked out environments

• git branch -m master main
• git fetch origin (or whatever your upstream branch is named)
• git branch -u origin/main main
• git remote set-head origin -a