[9.20] fix: usr: The resolver now removes other RRsets at the same name when caching...

When an RRset is in stale cache, and the authoritative server changes the record type to CNAME, the resolver fails to refresh the stale cache. This has been fixed.

Closes #5302

Backport of MR !11758

Merge branch 'backport-5302-serve-stale-cname-to-a-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12040