doc: Mark Minix as no longer tested. Rationale: No commits since 2018 at https://git.minix3.org/. Reported by Paul Eggert and Jim Meyering in <https://lists.gnu.org/archive/html/grep-devel/2022-07/msg00027.html>. * doc/gnulib-intro.texi (Supported Platforms): Mark Minix as no longer tested.