Use conflicts for vers spec of optional cairo2 dependency

Co-authored-by: Kate <[email protected]>

Author: geoffder

packages/OCADml/OCADml.0.5.0/opam

@@ -15,7 +15,10 @@ depends: [
   "odoc" {with-doc}
 ]
 depopts: [
-  "cairo2" {>= "0.6.2"}
+  "cairo2"
+]
+conflicts: [
+  "cairo2" {< "0.6.2"}
 ]
 build: [
   ["dune" "subst"] {dev}