Disable statement timeout during room purge (#18017)

This is already done for `purge_history` but seems to have been
forgotten for `purge_room`.

changelog.d/18017.misc

+Disable DB statement timeout when doing a purge room since it can be quite long.

synapse/storage/databases/main/purge_events.py

@@ -376,6 +376,11 @@ class PurgeEventsStore(StateGroupWorkerStore, CacheInvalidationWorkerStore):
                 (room_id,),
             )
 
+        if isinstance(self.database_engine, PostgresEngine):
+            # Disable statement timeouts for this transaction; purging rooms can
+            # take a while!
+            txn.execute("SET LOCAL statement_timeout = 0")
+
         # First, fetch all the state groups that should be deleted, before
         # we delete that information.
         txn.execute(