fix: usr: Negative caching stopped working with stale-answer-client-timeout 0

With "stale-answer-client-timeout 0" configured, every client query for a
name cached as NXDOMAIN or NODATA was sent on to the authoritative servers,
even while the cached negative answer was still within its TTL, so the
resolver effectively lost negative caching. Negative answers are now
refreshed only once they have actually gone stale.

Closes #6245

Merge branch '6245-fix-query_stale_refresh_ncache' into 'main'

See merge request isc-projects/bind9!12381