units: order systemd-user-sessions.service after home.mount

This should make /home as automount work reasonably well.

If /home is an automount this has little effect at boot, because if the
automount is not triggered it doesn't matter how the associated mount is
ordered.

It does matter at shutdown however, where home.mount is likely active
now. There the ordering means we'll end sessions first, and only then
deactivate home.mount.

Fixes: #16291