[9.18] fix: usr: Don't enforce NOAUTH/NOCONF flags in DNSKEYs

All DNSKEY keys are able to authenticate. The `DNS_KEYTYPE_NOAUTH` (and `DNS_KEYTYPE_NOCONF`) flags were defined for the KEY rdata type, and are not applicable to DNSKEY. Previously, however, because the DNSKEY implementation was built on top of KEY, the `_NOAUTH` flag prevented authentication in DNSKEYs as well. This has been corrected.

Closes #5240

Backport of MR !10261

Merge branch 'backport-5240-ignore-noauth-flag-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10316