Consider archiving this feedstock

I'm not sure of the history of why this feedstock exists, but I think we might want to consider archiving it. It packages a specific ABI version of `libidn`, which means it cannot be upgraded to a newer version of `libidn`. When I use `repoquery` to see what depends on this in conda-forge and bioconda, no results are returned. This is probably in no small part because `libidn` has been superseded by `libidn2`, as explicitly stated on the `libidn` homepage: https://www.gnu.org/software/libidn/. Furthermore, `libidn2` has a compatibility layer with `libidn`, and indeed we do have some packages that depend on `libidn2`. It's possible at some point we did have some packages that needed this specific ABI version of `libidn`, but it appears we no longer need it, and I'm thinking it would make sense to just archive this and redirect users to use `libidn2` instead.