Mark Dune 3.8.0 as incompatible with Coq < 8.13

Due to a bug, the new release of Dune 3.8.0 is incompatible with any Coq package
that depends on Coq < 8.13. See ocaml/dune#7846 for
details.
Even though Dune is technically compatible with these Coq versions itself, this
doesn't seems very useful. An older version of Dune will do perfectly well for
these use-cases and Dune 3.8.1 will be around soon. So I propose to mark a
conflict with these versions of Coq. Otherwise, we end up with a large number of
uninstallable packages with one particular version of Dune.

packages/dune/dune.3.8.0/opam

@@ -46,6 +46,7 @@ depends: [
   "base-unix"
   "base-threads"
 ]
+conflicts: [ "coq" {< "8.13"} ]
 url {
   src: "https://github.com/ocaml/dune/releases/download/3.8.0/dune-3.8.0.tbz"
   checksum: [