Make use of DynamicUser= and StateDirectory=
If umwelt-info/infrastruktur!22 removes the need to serve the crates.io index via rsync, we should be able to make use of systemd's DynamicUser=
and StateDirectory=
directives instead of manually managing user, group and directory. (Not sure if WorkingDirectory=$STATE_DIRECTORY
will work or whether we need to duplicate the naming.)