wait for current_state_events_membership before delete_old_current_state_events (#6924)

changelog.d/6924.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.10.0 which would cause room state to be cleared in the database if Synapse was upgraded direct from 1.2.1 or earlier to 1.10.0.

synapse/storage/data_stores/main/schema/delta/57/delete_old_current_state_events.sql

@@ -15,5 +15,8 @@
 
 -- Add background update to go and delete current state events for rooms the
 -- server is no longer in.
-INSERT into background_updates (update_name, progress_json)
-    VALUES ('delete_old_current_state_events', '{}');
+--
+-- this relies on the 'membership' column of current_state_events, so make sure
+-- that's populated first!
+INSERT into background_updates (update_name, progress_json, depends_on)
+    VALUES ('delete_old_current_state_events', '{}', 'current_state_events_membership');