Rename the default branch to main

[jtpio]

Most of the other Jupyter repositories have now switch to main as their default branch.

Opening this issue for visibility. If someone would like to go ahead and do the renaming feel free! Otherwise we can give it a couple of days until 0.7.0 stabilizes before doing it.

It should now be relatively straightforward using the GitHub UI, which will update the PR automatically:

We can then post a quick message here to let folks know how to update their forks, similar to jupyter/notebook#6295. And pin the issue to increase visibility and awareness about this change.

[jhamrick]

I've now renamed master to main. To update forks to have the main branch too:

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

[jtpio]

Nice, thanks!

[jtpio]

cc @brichet for awareness