Ocaml dependency unnecessarily high

The minimal OCaml version required is 4.12.0 but Debian stable (11, bullseye) currently has 4.11.1 which is enough to build the message-router. Can you please downgrade the requirement so that the message router will build on Debian stable?

https://gitlab.com/public.dream/upsycle/message-router/-/blob/master/upsycle-router.opam#L14