Matrix Spec 1.3 reduces metadata in encrypted messages

The release post specifies the spec changes.

I'm not entirely sure what the entirety of the change regarding metadata reduction is, but this MSC seems relevant:

• Deprecate the sender_key and device_id on m.megolm.v1.aes-sha2 events, and the sender_key on m.room_key_request to-device messages, as per MSC3700. (#1101)