This excludes the other stakes from the proof hash that is signed by a
stake's private key.
Previously the stake's key was signing the full ProofId, which was computed by hashing the other stakes.
This made it impossible to add additional stakes to a proof without knowing all the private keys for all the stakes, as adding stakes to a proof would change the data to be signed.
With this change, collaboratively building a proof is now made easier, as each stake can be signed without knowing all of the other stakes.
This change invalidates previously generated proofs, as the stake signatures will need to be updated.