ocamlPackages.ffmpeg: modernized derivation

- Remove `with lib;`
- Changed rev to tag, and sha256 to hash
- Added meta.changelog

Co-authored-by: Vincent Laporte <vbgl@users.noreply.github.com>

pkgs/development/ocaml-modules/ffmpeg/base.nix

@@ -6,14 +6,17 @@ rec {
   src = fetchFromGitHub {
     owner = "savonet";
     repo = "ocaml-ffmpeg";
-    rev = "refs/tags/v${version}";
-    sha256 = "sha256-MWO8B/L/KHLuq/BIIIidsLbFwGIwt/xj+/M1zEp8Z/8=";
+    tag = "v${version}";
+    hash = "sha256-MWO8B/L/KHLuq/BIIIidsLbFwGIwt/xj+/M1zEp8Z/8=";
   };
 
-  meta = with lib; {
+  meta = {
     homepage = "https://github.com/savonet/ocaml-ffmpeg";
     description = "Bindings for the ffmpeg libraries";
-    license = licenses.lgpl21Only;
-    maintainers = with maintainers; [ dandellion ];
+    changelog = "https://raw.githubusercontent.com/savonet/ocaml-ffmpeg/refs/tags/${src.tag}/CHANGES";
+    license = lib.licenses.lgpl21Only;
+    maintainers = with lib.maintainers; [
+      dandellion
+    ];
   };
 }