Debian and RPM: gwdatafind-server should install-depend on gunicorn

Since gwdatafind-server uses gunicorn it should come with an (install) dependency.

For Debian Buster, gunicorn3 is to be used (Python3); for Bullseye and later the package is called gunicorn, and the Bullseye one Provides: gunicorn but this may not be the final truth - so better have a dependency on gunicorn3 | gunicorn