Change ultrakeccak vk hash to be a poseidon2 hash instead of keccak hash

[lucasxia01]

We want to avoid having multiple different types of vk hashes. UltraKeccak just calls the Transcript hash, but we hopefully can change this to a poseidon2 hash without much issue.

[ledwards2225]

Doesn't seem worth the shake-up this late in the game