PSA: webauthn repo's "master" branch renamed "main"

PSA: https://github.com/w3c/webauthn's "master" branch has been renamed to
"main".

If you have a local clone of the webauthn repo, you can update it by
running:

cd <path to>/webauthn
git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a
<https://github.com/w3c/webauthn>