synapse/changelog.d/12709.removal

Require a body in POST requests to `/rooms/{roomId}/receipt/{receiptType}/{eventId}`, as required by the [Matrix specification](https://spec.matrix.org/v1.2/client-server-api/#post_matrixclientv3roomsroomidreceiptreceipttypeeventid). This breaks compatibility with Element Android 1.2.0 and earlier: users of those clients will be unable to send read receipts.