chore: Remove appspot references from source (#10015)

Issue https://github.com/shaka-project/shaka-player/issues/9984

docs/design/current/ui.md

@@ -28,4 +28,4 @@ castProxy.player == ui.player
 
 For details on adding Shaka UI to your application, see [UI library tutorial].
 
-[UI library tutorial]: https://uipreview-dot-shaka-player-demo.appspot.com/docs/api/tutorial-ui.html
+[UI library tutorial]: https://shaka-project.github.io/shaka-player/docs/api/tutorial-ui.html

lib/util/error.js

@@ -709,7 +709,7 @@ shaka.util.Error.Code = {
    * or containers are supported by the browser.
    *
    * To see what your browser supports, you can check the JSON data dumped by
-   * http://support.shaka-player-demo.appspot.com/
+   * https://shaka-project.github.io/shaka-player/support.html
    */
   'CONTENT_UNSUPPORTED_BY_BROWSER': 4032,