Update role to use new 2.15 webserver

In [version 2.15](https://github.com/paperless-ngx/paperless-ngx/releases/tag/v2.15.0) paperless [switched away from gunicorn to granian](https://github.com/paperless-ngx/paperless-ngx/pull/9218). Using this role fails with `Source /opt/paperless-ngx/gunicorn.conf.py not found` with that new paperless version.