[9.20] fix: dev: Fix the cache findzonecut() implementation

The search for the deepest known zone cut in the cache could improperly reject a node if it contained any stale data, regardless of whether it was the NS RRset that was stale.

Closes #5155

Backport of MR !10047

Merge branch 'backport-5155-fix-findzonecut-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10050