higher-bound constraint on ocaml version for mdx packages

[raphael-proust]

There have been some errors in the revdeps of the CI of some other packages pointing to mdx issues. This PR adds missing upper-bound constraints.

[kit-ty-kate]

Example of such error:

# File "lib/top/mdx_top.ml", lines 217-220, characters 27-44:
# Warning 8 [partial-match]: this pattern-matching is not exhaustive.
# Here is an example of a case that is not matched:
# Pextra_ty (_, _)

Fixed upstream by realworldocaml/mdx#435

[mseri]

Thanks

[talex5]

Marking MDX as incompatible with OCaml 5.1 means that projects using MDX are not getting much testing in ocaml-ci. For example, Eio is now only tested on one platform (debian-12-5.0_opam-2.1). All the other distributions and architectures try to test with 5.1, which is no longer available.

[mseri]

We need to push for a release of mdx with the fix. I have opened an issue on the repo