admin/purgeRoom requires server restart

*This issue was originally created by [**@bones-was-here**](https://github.com/bones-was-here) at .*

### Background information

- **Dendrite version or git SHA**: 0.13.2
- **SQLite3 or Postgres?**: postgres
- **Running in Docker?**: no
- **`go version`**: 1.21
- **Client used (if applicable)**: element web

### Description

- **What** is the problem: After purging a room, dendrite must be restarted before the room can be rejoined
- **Who** is affected: dendrite admins
- **How** is this bug manifesting: When attempting to rejoin the room, a big spam of errors is logged and the join fails.
- **When** did this first appear: unknown

### Steps to reproduce

- evacuate/leave and then purge a room with other servers participating
- rejoin the room without first restarting dendrite

Start at the admin/purgeRoom entry point and reproduce the reported sequence with PostgreSQL: evacuate or leave a room, purge it with other servers participating, then rejoin without restarting Dendrite. Trace the resulting errors and compare the behavior after a restart; done means the room can be rejoined without restarting and without the reported error spam.

Written by the indexing model from the issue text.