Synapse doesn't tell clients when they can't change passwords due to SSO

This issue has been migrated from [#11944](https://github.com/matrix-org/synapse/issues/11944).

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I filed this over in Element Web land: https://github.com/vector-im/element-web/issues/19461 - the problem is that for accounts using SSO (in my case using github), the UX gives you the option to change your password, but that's not _really_ possible.

@t3chguy I think is saying that the problem is that Synapse still returns:

```
"m.change_password": {
"enabled": true
}
```

for SSO accounts, when that should be false.