Disable Postgres statement timeouts while purging room state

[realtyem]

I tried to delete and purge #matrix:matrix.org from my server. The miscellaneous room data was deleted just fine. But I found that there was state still hanging around, then discovered this in my logs:

2025-01-28 21:39:56,756 - synapse.storage.txn - 787 - WARNING - task-shutdown_and_purge_room-0-ruJgsVyacLWdLRiW-!OGEhHVWSdvArJzumhm:matrix.org - [TXN OPERROR] {purge_room_state-136b} canceling statement due to statement timeout
 0/5
2025-01-28 22:39:57,386 - synapse.storage.txn - 787 - WARNING - task-shutdown_and_purge_room-0-ruJgsVyacLWdLRiW-!OGEhHVWSdvArJzumhm:matrix.org - [TXN OPERROR] {purge_room_state-136b} canceling statement due to statement timeout
 1/5
2025-01-28 23:39:58,688 - synapse.storage.txn - 787 - WARNING - task-shutdown_and_purge_room-0-ruJgsVyacLWdLRiW-!OGEhHVWSdvArJzumhm:matrix.org - [TXN OPERROR] {purge_room_state-136b} canceling statement due to statement timeout
 2/5
2025-01-29 00:39:59,486 - synapse.storage.txn - 787 - WARNING - task-shutdown_and_purge_room-0-ruJgsVyacLWdLRiW-!OGEhHVWSdvArJzumhm:matrix.org - [TXN OPERROR] {purge_room_state-136b} canceling statement due to statement timeout
 3/5
2025-01-29 01:40:01,788 - synapse.storage.txn - 787 - WARNING - task-shutdown_and_purge_room-0-ruJgsVyacLWdLRiW-!OGEhHVWSdvArJzumhm:matrix.org - [TXN OPERROR] {purge_room_state-136b} canceling statement due to statement timeout
 4/5
2025-01-29 02:40:03,991 - synapse.storage.txn - 787 - WARNING - task-shutdown_and_purge_room-0-ruJgsVyacLWdLRiW-!OGEhHVWSdvArJzumhm:matrix.org - [TXN OPERROR] {purge_room_state-136b} canceling statement due to statement timeout
 5/5
2025-01-29 02:40:04,013 - synapse.util.task_scheduler - 442 - ERROR - task-shutdown_and_purge_room-0-ruJgsVyacLWdLRiW - scheduled task ruJgsVyacLWdLRiW failed

After making this change, the actual data took 1.18 hours to actually finish.
This is done for _purge_rooms_txn(), feels right it should be over here too.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file.
• Code style is correct
  (run the linters)