fix(plex): restart service after update to apply new version

After apt install updates the plexmediaserver package, the running
service needs an explicit restart so the new version is loaded.
Without this, users see the update succeed but Plex still shows
the old version in the web interface.

Ref #12993

ct/plex.sh

@@ -79,6 +79,11 @@ function update_script() {
   $STD apt update
   $STD apt install -y plexmediaserver
   msg_ok "Updated Plex Media Server"
+
+  msg_info "Restarting Plex Media Server"
+  systemctl restart plexmediaserver
+  msg_ok "Restarted Plex Media Server"
+
   msg_ok "Updated successfully!"
   exit
 }