Raise minimum rresult version

The code needs at least version 0.2.0 of rresult for `>>=` and it needs
at least 0.3.0 for `rresult` to use the `result` compatibility package
to define a `result` type that is compatible with the stdlib `result`
type.

Discovered via the minimum-version checks of
ocaml/opam-repository#21475

graphql.opam

@@ -16,7 +16,7 @@ depends: [
   "dune" {>= "1.1"}
   "graphql_parser" {>= "0.9.0"}
   "yojson"
-  "rresult"
+  "rresult" {>= "0.3.0"}
   "seq"
   "alcotest" {with-test}
 ]