[CVE-2026-1519] sec: usr: Fix unbounded NSEC3 iterations when validating referrals...

DNSSEC-signed zones may contain high iteration-count NSEC3 records,
which prove that certain delegations are insecure. Previously, a
validating resolver encountering such a delegation processed these
iterations up to the number given, which could be a maximum of 65,535.
This has been addressed by introducing a processing limit, set at 50.
Now, if such an NSEC3 record is encountered, the delegation will be
treated as insecure.

ISC would like to thank Samy Medjahed/Ap4sh for bringing this
vulnerability to our attention.

Closes isc-projects/bind9#5708

Merge branch '5708-confidential-nsec3-delegation-iteration-fix-fallback-to-insecure' into 'v9.21.20-release'

See merge request isc-private/bind9!935