Elixir 1.20 requires Erlang/OTP 27 or newer, so tighten BUILD_DEPENDS
from erlang>26 (which an OTP 26.2.x package still satisfies) and refresh
pkg-message accordingly.
ex_doc now writes .build.epub and .build.markdown markers next to the
old .build one, so widen the post-extract cleanup pattern to keep them
out of DOCSDIR.