Do not support OCaml 5.1

There is some behavior change that needs to be investigated.

Co-authored-by: Anil Madhavapeddy <[email protected]>

Author: Leonidas-from-XIV

packages/mdx/mdx.2.3.0/opam

@@ -18,7 +18,7 @@ homepage: "https://github.com/realworldocaml/mdx"
 bug-reports: "https://github.com/realworldocaml/mdx/issues"
 depends: [
   "dune" {>= "3.5"}
-  "ocaml" {>= "4.08.0"}
+  "ocaml" {>= "4.08.0" & < "5.1.0"}
   "ocamlfind"
   "fmt" {>= "0.8.7"}
   "cppo" {build & >= "1.1.0"}