Setting room history to anyone exposes users to search

This issue has been migrated from [#9202](https://github.com/matrix-org/synapse/issues/9202).

---

### Description

Setting history in a private room to "Anyone" exposes users to search although `search_all_users: false` .

### Steps to reproduce

- Alice and Bob are on server, but don't know each other
- Bob can't find Alice in user search
- Alice creates a private room
- Alice sets "Who can read history?" to "Anyone"
- Bob can find Alice in user search

Resetting the history to "Members only (since the point in time of selecting this option)" restores the original behavior.

Done with Element 1.7.17.

### Version information

- **Homeserver**: Non-public homeserver

- **Version**: {"server_version":"1.24.0","python_version":"3.8.6"}

- **Install method**: https://github.com/spantaleev/matrix-docker-ansible-deploy/

- **Platform**: Docker

### Settings
```yaml
#user_directory:
# enabled: true
# search_all_users: false
```
or
```yaml
user_directory:
enabled: true
search_all_users: false
```