Default branch changed from master to main

[bradlitterell]

Consistent with community expecations and github guidance, the default branch of microsoft/ms-tpm-20-ref has been changed from master to main. This will not update a user's local environment. The following commands can be used to update your local environment to match:

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