Change default NSEC3 iterations to 0 also for dnssec-signzone

We have already changed default NSEC3 iterations value for dnssec-policy in MR !5513 (merged), but we forgot to update dnssec-signzone to use the same default.

Currently dnssec-signzone in BIND 9.19.1 still defaults to 10 iterations, which is not in line with current recommendations.

I propose to treat this as omission in !5513 (merged) and fix it even for v9.18 . Sooner the better before lots of people migrate.