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