fix: dev: Don't serve a stale CNAME or record when fresh data of the other exists

When a cached name held both a CNAME and records of another type — one stale,
the other still fresh — named with serve-stale could return the expired set
instead of the fresh one, in either direction. It now prefers whichever is fresh.

Merge branch 'ondrej/fix-serve_stale-cname-and-type' into 'main'

See merge request isc-projects/bind9!12282