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

security/nacl/Makefile

@@ -11,6 +11,9 @@ COMMENT=	High speed cryptography library
 
 LICENSE=	PD
 
+DEPRECATED=	Unmaintained by upstream, use libsodium instead
+EXPIRATION_DATE=2021-12-31
+
 USES=		tar:bzip2
 USE_BINUTILS=	yes
 MAKE_JOBS_UNSAFE=	yes