math/djbfft: Deprecate and set expiration date to 2023-09-30

Like most of D. J. Bernstein's other projects most of them have been
untouched for a very long time and the license also prevents us from
distributing it. Compared to other libraries it's also quite slow and
we have no consumers in tree so retire it.

Reference: https://www.spek.cc/p/faster-fast-fourier-transform

math/djbfft/Makefile

@@ -14,6 +14,9 @@ LICENSE_NAME=	djb
 LICENSE_TEXT=	No license -- see http://cr.yp.to/softwarelaw.html
 LICENSE_PERMS=	dist-mirror dist-sell pkg-mirror pkg-sell auto-accept
 
+DEPRECATED=	Abandonware, slow and untouched by upstream for over 20 years
+EXPIRATION_DATE=2023-09-30
+
 DISABLE_SIZE=	yes
 ALL_TARGET=
 INSTALL_TARGET=setup check