Fix build errors and update ocamlformat versions

- ocaml-net/lib/http.ml: Fix Eio.Flow type compatibility for cohttp-eio
- ocaml-net/fuzz/fuzz_net.ml: Fix unused variable warning
- kdf/*/tests/dune: Add (modes native) to avoid js_of_ocaml requirement
- ocaml-matter/dune-project: Remove pins (packages in monorepo)
- Update .ocamlformat versions to 0.28.1 across packages
- ocaml-meross: Various improvements