Update gate_challenges for constant honk proof size

[ledwards2225]

Currently we generate log_n many gate challenges (the challenges used to construct the pow polynomial for sumcheck). For consistency with the constant honk proof size work, this would need to be updated in accordance with CONST_PROOF_SIZE_LOG_N. This dependence on the log of the circuit size does not currently effect the constant proof size / constant recursive verifier circuit work since at the time of writing this constraints are not generated for hashing in the the Ultra recursive verifier. Until hashing produces constraints, it is difficult to test any updates to this code.