Fix error when compiling some circuits

[xevisalle]

Describe the bug
When compiling some circuits, we get the error TruncatedDegreeTooLarge. The error is catched from the function truncate() in src/commitment_scheme/kzg10/key.rs, but this bug comes from srs.rs, where the max_degree in setup() should be + 6. Also, the truncated degree used in trim() should be + 6.

The explanation is that adding the blinding factors requires some extra elements for the SRS: +1 per each wire (we have 4 wires), plus +2 for the permutation polynomial.

To Reproduce
Among others, the gadget schnorr crashes when executing the double key verification. It can be reproduced by cloning the repo, selecting the last version of plonk, and using cargo bench or cargo test.

Expected behaviour
Pass all the tests and circuits with no errors.