Reject attempts to change the device keys of an existing device

[richvdh]

Synapse allows clients to change the Ed25519 and Curve25519 keys of an existing device via /keys/upload. I don't think this has a legitimate usecase; indeed matrix-sdk-crypto ignores any attempt to change the Ed25519 key.

And it's not great for performance, especially if you have a couple of clients fighting over the "correct" key.

[Note that resetting your cross-signing keys requires changing the signature on the device keys, but not the keys themselves.]

[richvdh]

Vaguely related: #11720

[BillCarsonFr]

There is an existing meta issue for it on meta element-hq/element-meta#2374

Will link this issue from it