opam: Require coq < 8.16

`entree-specs` is not currently buildable with Coq 8.16 or later. See #1. As
such, we prevent these versions of Coq from being included in build plans by
excluding them at the `opam` file level.

opam

@@ -10,7 +10,7 @@ bug-reports: "https://github.com/GaloisInc/entree-specs/issues"
 depends: [
   "ocaml"
   "ocamlfind"
-  "coq" {>= "8.15"}
+  "coq" {>= "8.15" & < "8.16"}
   "coq-itree" {>= "5.2" & < "5.3"}
   "coq-paco" {>= "4.1.2"}
 ]