[9.20] fix: usr: Reject RRSIG records covering meta-types

A recursive resolver could accept and cache an RRSIG record whose
Type-Covered field names a meta-type (ANY, AXFR, IXFR, MAILA, MAILB),
even though no real RRset of those types ever exists. Such records
are now rejected by the DNS message parser.

Closes #6002

Backport of MR !12048

Merge branch 'backport-6002-reject-rrsig-covering-meta-types-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12051