feat(@schematics/angular): hide universal schematic

The universal schematic by itself doesn't get you a working server application.

BREAKING CHANGE: universal schematic is now hidden and cannot be used with the `ng generate` command.

Users who want a working universal application should use `ng add @nguniversal/express-engine` or `ng add @nguniversal/hapi-engine`

Closes: #15166

Author: alan-agius4

packages/schematics/angular/collection.json

@@ -87,7 +87,8 @@
     "universal": {
       "factory": "./universal",
       "description": "Create an Angular universal app.",
-      "schema": "./universal/schema.json"
+      "schema": "./universal/schema.json",
+      "hidden": true
     },
     "appShell": {
       "aliases": [ "app-shell" ],