Use OpenID Connect instead of OAuth2 for XWiki Connection on weblate

We currently use a custom implementation of OAuth2 for the connection of Weblate using XWiki. See: https://github.com/xwiki/xwiki-dev-tools/blob/master/weblate-scripts/auth/xwiki.py
It would be better for maintainability to use directly OpenID Connect implementation for this, see: https://python-social-auth.readthedocs.io/en/latest/backends/openid.html