mark openapi.core/openapi-spec as deprecated

see comment for details

src/schema_tools/openapi/core.cljc

@@ -454,7 +454,12 @@
 ;; Generate the OpenAPI spec
 ;;
 
-(defn openapi-spec
+;; Top-level openapi spec generation was moved to reitit in
+;; https://github.com/metosin/reitit/pull/638
+;;
+;; Once reitit-0.7.0-alpha6 has been out for some time, this can be
+;; deleted since it should have no other users.
+(defn ^:deprecated openapi-spec
   "Transform data into an OpenAPI spec. Input data must conform to the Swagger3
   Spec (https://swagger.io/specification/) with a exception that it can have
   any qualified keywords which are expanded with the