Lower-bound ppx_deriving to 6.1.0

Author: jmid

ppx_deriving_qcheck.opam

@@ -12,7 +12,7 @@ depends: [
   "ocaml" {>= "4.08.0"}
   "qcheck-core" {>= "0.24"}
   "ppxlib" {>= "0.36.0"}
-  "ppx_deriving" {>= "5.2.1"}
+  "ppx_deriving" {>= "6.1.0"}
   "odoc" {with-doc}
   "alcotest" {with-test & >= "1.4.0" }
   "qcheck-alcotest" {with-test & >= "0.24"}