Valid `m.room.tombstone` state event results in "Content has no replacement_room key" error

### Description

When trying to send an `m.room.tombstone` custom state event through either `devtools` or Swagger, I only receive an `Content has no replacement_room key` error despite the `replacement_room` being populated in the `content`.

devtools

```json
{
"content": {
"replacement_room": "!dwCfBTJakfZKJONWKw:matrix.org",
"body": "This room has been replaced"
},
"room_id": "!GGpVnIHTefZpWGatgK:hackingfor.eu",
"sender": "@thomcat:thomcat.rocks"
}
```

Swagger

```json
{
"content": {
"body": "This room has been replaced",
"replacement_room": "!dwCfBTJakfZKJONWKw:matrix.org"
},
"room_id": "!GGpVnIHTefZpWGatgK:hackingfor.eu",
"sender": "@thomcat:thomcat.rocks",
"state_key": "",
"type": "m.room.tombstone"
}
```

### Steps to reproduce

- `devtools` -> Explore room state -> Send custom state event
- https://playground.matrix.org/#put-/_matrix/client/v3/rooms/-roomId-/state/-eventType-/-stateKey- -> Paste JSON into "Example" -> Click "Try" button

### Homeserver

thomcat.rocks

### Synapse Version

1.121.1

### Installation Method

Docker (matrixdotorg/synapse)

### Database

Single Postgres, never ported from SQLite, never restored previously

### Workers

Single process

### Platform

Ubuntu 22.04, running `synapse` in docker container

### Configuration

_No response_

### Relevant log output

```shell
synapse.http.server - 130 - INFO - PUT-173 - SynapseError: 400 - Content has no replacement_room key
```

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

_No response_