Chore: Explicitly call Stdlib.Mutex in file with open Base (ocaml#9929)

* Explicitly call Stdlib.Mutex in file with open Base

Signed-off-by: Ambre Austen Suhamy <[email protected]>

* Silence deprecation alert with note to upgrade

Signed-off-by: Ambre Austen Suhamy <[email protected]>

---------

Signed-off-by: Ambre Austen Suhamy <[email protected]>

Author: ElectreAAS

test/expect-tests/dune_file_watcher/dune_file_watcher_tests_lib.ml

@@ -4,6 +4,12 @@ open Base
 open Stdune
 
 let critical_section mutex ~f =
+  (* Since 5.0, using "Mutex" with Base open rings an alert and suggests
+     we use "Stdlib.Mutex" instead.
+     Prior to OCaml 5.0, "Stdlib.Mutex" didn't exist, it was just "Mutex".
+     Since 5.1 there is Stdlib.Mutex.protect which replaces this function.
+  *)
+  let module Mutex = Mutex [@alert "-deprecated"] in
   Mutex.lock mutex;
   Exn.protect ~f ~finally:(fun () -> Mutex.unlock mutex)
 ;;