Renaming the default branch to main

[cookpa]

Following upstream changes in ITK and ANTs

[cookpa]

The default branch has been renamed!
master is now named main

If you have a local clone, you can update it by running the following commands.
git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

[ntustison]

Hey, thanks a lot @cookpa