This puts all gvfs systemd services (which are dbus-activated through
SystemdService=) inside of the
session scope that was introduced in v249 (https://github.com/systemd/systemd/pull/16603) and described in https://systemd.io/DESKTOP_ENVIRONMENTS/
This allows the daemons to be grouped with other session daemons so that, for example, higher priorities can be set on them through cgroup controllers.
Note that it still works on older systemd (the slice, which is just a cgroup inner directory, is created on demand)