Try start_background_tasks in synapse/_scripts/update_synapse_database.py so portdb can maybe finish

See #18886 (comment)

Author: MadLittleMods

synapse/_scripts/update_synapse_database.py

@@ -120,6 +120,10 @@ def main() -> None:
     # DB.
     hs.setup()
 
+    # TODO: Why do we need this here?
+    if hs.config.worker.run_background_tasks:
+        hs.start_background_tasks()
+
     if args.run_background_updates:
         run_background_updates(hs)