Depend on ocaml 3.12.1

PR that should close https://forge.ocamlcore.org/tracker/index.php?func=detail&aid=1318&group_id=54&atid=294

About the changes in ```MyOCamlbuildFindlib```, I didn't removed it globally since I don't want to break the compatibility with older oasis versions. I removed only the things that are included in ocaml 3.12.1 and newer.