security/nacl: Deprecate and set expiration date to 2021-12-31

No new release for over 10 years and pretty much "any" user of NaCl have
migrated to security/libsodium or at least really should by now.
We have no users left in ports except for dns/djbdns which is "half" broken
and support isn't enabled by default.

PR: 256449
Approved by: port maintainer timeout (3 months), arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D32215