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