[9.20] fix: usr: Fix NTA (Negative Trust Anchor) expiration issue

When a configured NTA for a name expired, any possibly cached
data for the name (with "insecure" DNSSEC validation result)
was not flushed from the resolver's cache. This has been fixed.

Closes #5747

Backport of MR !11597

Merge branch 'backport-5747-nta-expiry-cache-flush-bug-fix-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11715