DEV: Only add ember-global deprecation for Ember 3 (#24667)

(otherwise, we're accidentally extending support into Ember 5!)

Followup to 106c1c317f

app/assets/javascripts/ember-production-deprecations/vendor/ember-production-deprecations/deprecate-shim.js

@@ -35,6 +35,7 @@ define("discourse/lib/deprecate-shim", ["exports"], function (exports) {
     );
 
     // Patch ember-global deprecation
+    if (window.hasOwnProperty("Ember")) {
       Object.defineProperty(window, "Ember", {
         enumerable: true,
         configurable: true,
@@ -56,6 +57,7 @@ define("discourse/lib/deprecate-shim", ["exports"], function (exports) {
           return require("ember").default;
         },
       });
+    }
   };
 });