CI: Change master -> main
Change needed to get GHA and Azure running again after the branch renaming.

I also opened https://github.com/pandas-dev/pandas/pull/45336, which is a more complete replace (also updates the contributing docs). Now, if that needs more review (since it's bigger), it's also fine to merge this first quickly to get CI running again (it shouldn't give conflicts, as this PR should be a strict subset of the other) > I also opened #45336, which is a more complete replace (also updates the contributing docs). Now, if that needs more review (since it's bigger), it's also fine to merge this first quickly to get CI running again (it shouldn't give conflicts, as this PR should be a strict subset of the other)

Ah sorry I didn't notice this PR. Closing this one.