`enable_set_avatar_url` not honoured for first login

### Description

This is in reference to: https://github.com/element-hq/element-web/issues/30567

1. Set `enable_set_avatar_url` to `false` in the `homeserver.yaml` for the Matrix ecosystem
2. Login as user in fresh Matrix ecosystem (in this case with Element Web), you see "letter" (B for example) in the middle of the welcome page after logging in.
3. Click on it and it asks for a picture, select picture, it uploads and overrides your avatar when it was disabled at the Matrix Homeserver level

The Element Web client seems to have a window where it can override the "enable_set_avatar_url" value being set to "false" in homeserver.yaml

This matters because one of the implementations I'm working on requires the user avatars be pulled from external authentication (LDAP/AD/SSO/etc) and users be blocked from changing their avatar within Matrix/clients.

### Steps to reproduce

Outlined above

### Homeserver

internal self-hosted

### Synapse Version

1.136.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

PostgreSQL 17

### Workers

Single process

### Platform

Container, outlined above

### Configuration

_No response_

### Relevant log output

```shell
I could not identify relevant log entries for this being overriden
```

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

_No response_