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.

nixos/modules/system/boot/plymouth.nix

@@ -87,9 +87,10 @@ in
 
     systemd.services.plymouth-kexec.wantedBy = [ "kexec.target" ];
     systemd.services.plymouth-halt.wantedBy = [ "halt.target" ];
+    systemd.services.plymouth-quit-wait.wantedBy = [ "multi-user.target" ];
     systemd.services.plymouth-quit = {
       wantedBy = [ "multi-user.target" ];
-      after = [ "display-manager.service" "multi-user.target" ];
+      after = [ "display-manager.service" ];
     };
     systemd.services.plymouth-poweroff.wantedBy = [ "poweroff.target" ];
     systemd.services.plymouth-reboot.wantedBy = [ "reboot.target" ];