Build benchmarks only for OCaml >= 4.14.0

Author: cknitt

tests/syntax_benchmarks/dune

@@ -9,6 +9,7 @@
  (enabled_if
   (and
    (<> %{profile} browser)
+   (>= %{ocaml_version} "4.14.0")
    (or
     (= %{system} macosx)
     ; or one of Linuxes (see https://github.com/ocaml/ocaml/issues/10613)