nixos/plymouth: multi-user.target wants plymouth-quit-wait.service

This is apparent from the service file directory in plymouth:
├── multi-user.target.wants
│   ├── plymouth-quit.service -> ../plymouth-quit.service
│   └── plymouth-quit-wait.service -> ../plymouth-quit-wait.service

Leaving it unspecified caused gdm-wayland to crash on boot, see #39615.

The change made other display managers not quit plymouth properly however. By
removing "multi-user.target" from `plymouth-quit.after` this is resolved.