GDM now specifies ordering between `plymouth-quit` and `display-manager`:
9be5321097c16
This causes an ordering cycle between GDM and plymouth-quit which can result in
systemd breaking GDM:
```
plymouth-quit.service: Job display-manager.service/start deleted to break
ordering cycle starting with plymouth-quit.service/start
```
Not sure how often this triggers, as I've run my system with plymouth and
9be5321097c16 without any issues. But I did catch a VM doing this.
NOTE: I also tried to remove the ordering in GDM to see if plymouth managed to
live longer, but it didn't seem to help. So I opted to stick as close to
upstream (upstream GDM specifies ordering, but plymouth does not).
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