"Terms and Conditions" uses wrong URL

### Steps to reproduce

1. Setup `user_consent` according https://matrix-org.github.io/synapse/latest/consent_tracking.html
2. Homeserver `public_baseurl` is `https://matrix.example.com` and `server_name` is `example.com`
4. `/.well-known/matrix/client` content is...
```
{
"m.homeserver": {
"base_url": "https://matrix.example.com"
}
}
```
`/.well-known/matrix/server` contains `{ "m.server": "matrix.example.com:443" }`
5. Terms and Conditions from Legals menu shows then _This is not a valid Matrix server address_ because it sends request to root domain `POST https://example.com/_matrix/client/r0/register` instead of matrix subdomain `POST https://matrix.example.com/_matrix/client/r0/register`

### Outcome

#### What did you expect?
See correct URL in Legals

#### What happened instead?
_This is not a valid Matrix server address_ error message.

### Your phone model

Samsung Galaxy S20 FE

### Operating system version

Android 12

### Application version and app store

1.5.12

### Homeserver

self hosted Synapse 1.73.0

### Will you send logs?

No

### Are you willing to provide a PR?

No