Renamed default branch to main

[jtpio]

Anyone who has the repository already cloned will likely need to update it. These are the commands Github suggests:

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

[jtpio]

See #6293 and jupyter/notebook-team-compass#6 for more info.

[jtpio]

Closing as the branch was renamed a couple of months ago already.