"enable_set_displayname: false" still allows users to change their local room display name

### Description

Hello all. I bring to you today an oddly specific issue.

I am running a Synapse server (v1.103.0) and Element-Web frontend (v1.11.62) both via docker images. Users log into the environment using their LDAP credentials.

We decided it would be best to set **"enable_set_displayname"** to **false** in order to keep users using the LDAP provided display names.

While the setting does disable the ability for users to set their display name globally (/nick), they can still set their room display name (/myroomnick).

My initial expectation is that the setting would disable user abilities to use either of the commands that change their display name. When that wasn't the case, I was thinking I missed another setting to disable the (/myroomnick) option, but that doesn't seem to be the case.

I am unsure as to the best path forward. I suppose if the "enable_set_displayname" is set to false, most people would expect that to disable custom display names completely. But, in the interest of more customization maybe it could be another setting to disable the (/myroomnick).

If it turns out I missed an existing option, I'm sorry you wasted your time reading this.

### Steps to reproduce

- Build a fresh synapse & element-web environment
- Deploy successful ldap configuration
- add **enable_set_displayname: false** to homeserver.yaml
- have a non-admin user use /myroomnick to set a custom display name for the room

### Homeserver

private homeserver

### Synapse Version

1.103.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

- PostgresSQL v16.2, single server, no I have not ported, no I have not restored from a backup.

### Workers

Single process

### Platform

Synapse is running on a RHEL 8.9 VM under a ESXi hypervisor.
The containers are running via docker-ce-25.0.3-1.el8.x86_64, using docker compose.

### Configuration

-Server Notices
-User Consent
-ldap_auth_provider.LdapAuthProviderModule
-auto_join_rooms
-enable_set_displayname: false
-enable_3pid_changes: false

### Relevant log output

```shell
Unfortunately I can not provide logs at this time. This is an air-gapped system.
```

### Anything else that would be useful to know?

I guess this might be considered a feature request instead of a bug? I'm sorry I'm not sure how to categorize it, bug seemed like the place to start. If I need to recreate this as a feature request please let me know.