[9.20] rem: dev: Remove obsolete KEY record EXTENDED flag deprecated by RFC 3445

KEY resource records originally defined EXTENDED flag that was removed
by RFC 3445 back in 2002. BIND still carried code to parse and emit it,
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

Partial backport of MR !11961

Merge branch 'backport-5900-remove-keyflag-extended-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11962