rem: dev: Remove obsolete KEY record flags deprecated by RFC 3445

KEY resource records originally defined NOAUTH, NOCONF, EXTENDED, and
ENTITY flags that were removed by RFC 3445 back in 2002. BIND still
carried code to parse and emit them, including the additional two-octet
flags field that followed when the EXTENDED bit was set. That handling
has been removed and the affected bit positions are now reserved.

Dropping the extended-flags handling also eliminates a possible crash
that could be reached when signing a zone containing an invalid key.

Closes #5900

Merge branch '5900-remove-keyflag-extended' into 'main'

See merge request isc-projects/bind9!11961