etc: Systemd unit should declare after=multiuser.target (fixes #5346) (#5593)

etc/linux-systemd/system/[email protected]

@@ -1,7 +1,7 @@
 [Unit]
 Description=Syncthing - Open Source Continuous File Synchronization for %I
 Documentation=man:syncthing(1)
-After=network.target
+After=multi-user.target network.target
 
 [Service]
 User=%i