Downgrade Gunicorn to 21.2.0
This fixes a problem we currently encounter where running HELIPORT under a prefixed URL, e.g. https://heliport.example.com/app/, causes HTTP 404 errors.
This fixes a problem we currently encounter where running HELIPORT under a prefixed URL, e.g. https://heliport.example.com/app/, causes HTTP 404 errors.