Unspecced `redacted_by` field in `unsigned`

This issue has been migrated from [#8764](https://github.com/matrix-org/synapse/issues/8764).

---

Ran into the following event today:

```js
{
"content": {
"membership": "join"
},
"origin_server_ts": 1595256121167,
"sender": "@redacted_user_1:example.com",
"state_key": "@redacted_user_1:example.com",
"type": "m.room.member",
"unsigned": {
"redacted_by": "$redacted_id_1",
"redacted_because": {
"content": {},
"origin_server_ts": 1595261803914,
"redacts": "$redacted_id_2",
"sender": "@redacted_user_2:example.com",
"type": "m.room.redaction",
"unsigned": {
"age": 23764322
},
"event_id": "$redacted_id_1"
}
},
"event_id": "$redacted_id_2"
}
```

This event contains a `redacted_by` property in the `unsigned` data, but the client-server spec makes no mention of such a field, it only mentions `redacted_because`.