ocamlPackages.ppx_derivers: use Dune 3

pkgs/development/ocaml-modules/ppx_derivers/default.nix

···
-{ lib, fetchFromGitHub, buildDunePackage, ocaml }:
+{ lib, fetchFromGitHub, buildDunePackage }:
 
-buildDunePackage (rec {
+buildDunePackage rec {
   pname = "ppx_derivers";
   version = "1.2.1";
 
···
     maintainers = [ lib.maintainers.vbgl ];
     inherit (src.meta) homepage;
   };
-} // lib.optionalAttrs (!lib.versionAtLeast ocaml.version "4.08") {
-  duneVersion = "1";
-})
+}