flambda-backend: Use 4.14.0 opam switch for building (includes upgrad…

…ing ocamlformat to 0.24.1) (ocaml-flambda#1030)

HACKING.jst.adoc

@@ -2,9 +2,9 @@
 
 ocaml-jst has a dune-based build system which is different from
 upstream OCaml. To get started, you'll need a working install of OCaml
-4.12 and dune, e.g. via OPAM:
+4.14 and dune, e.g. via OPAM:
 
-    $ opam switch create 4.12.0
+    $ opam switch create 4.14.0
     $ eval $(opam env)
     $ opam install dune
 
@@ -30,7 +30,7 @@ to build without running the testsuite, or:
 to start a continuously polling build of the compiler.
 
 Note that the dune-based build system is entirely separate from the main
-Makefile, and so you will need to use `make -f Makefile.jst install` 
+Makefile, and so you will need to use `make -f Makefile.jst install`
 to install after building.
 
 ## Testing