Lower values of `fixed_log2_rows` for the wrap machine

[wwared]

The values for fixed_log2_rows here were decreased from 20 to 19 upstream v1.0.6-testnet (and again at v1.0.7-testnet), but this breaks the test test_permutation_challenges in Sphinx, so we kept it at 20

Setting it to 19 does not break E2E Plonk proof generation however, in fact with it at 20 it prints the warning "WARN fixed log2 rows can be potentially reduced: got 394238, expected 1048576". We should double-check why test_permutation_challenges is failing with a lower size

Running a quick test benchmark with 19 vs 20 showed no meaningful difference in total proving time, but this number changes how big the final gnark circuit is, so we'll want to decrease it as much as possible