rem: usr: Remove -C option from dnssec-keygen and dnssec-keyfromlabel

The -C option, introduced in BIND 9.7, caused a backward-compatible
key to be generated, using private key format version 1.2, omitting the
creation date and other timing metadata. This made it possible to
generate keys that could be loaded by older versions of BIND.

Those older versions having reached end of life many years ago, the
option can now be removed, along with the `dnssec-settime -f` option,
which caused old-style keys to be upgraded.

Merge branch 'each-remove-keygen-compat' into 'main'

See merge request isc-projects/bind9!11446