[OIDC] Auto-provision usernames from 'preferred_username' claim instead of 'name'

Just to let you know that while I agree that the username should use the prefered_username claim and the name the given_name (it seems to currently use it for both), users can actually set their password (and therefore change their username) even when SSO provisionned. If "require SSO" is not enabled they can even login with their email/password. Only thing that change is 2FA being bypassed by SSO.