Check correct copyright dates in man pages

In 2021 copyright updates (v9.16: !4519 (merged) & ~"v9.17": !4518 (merged)) we forgot to update copyright date in man pages, e.g. in doc/man/dnssec-keygen.1in:

.SH COPYRIGHT
2020, Internet Systems Consortium

We did not forget to update it in ~"v9.11" !4536 (merged) because util/update_copyrights of misc job would not let me (https://gitlab.isc.org/isc-projects/bind9/-/jobs/1390734).

v9.16 and ~"v9.17" should be updated to check for correct copyright dates in man pages (though the actual problem is in doc/man/conf.py).