Rewrite mount engine syntax

To disable deprecation warning since Rails 8.0.

```
DEPRECATION WARNING: Mounting an engine with a hash key name is deprecated and will be removed in Rails 8.1. Please use the at: option instead. Instead of: mount(SomeRackApp => "some_route") Please use: mount SomeRackApp, at: "some_route" (called from mount_ember_assets at /Users/ryunosuke.sato/src/github.com/thoughtbot/ember-cli-rails/lib/ember_cli/route_helpers.rb:40)
```

lib/ember_cli/route_helpers.rb

@@ -34,7 +34,7 @@ def mount_ember_assets(app_name, to: "/")
         app = ::EmberCli[app_name]
 
         if app.mountable?
-          mount app.to_rack => to
+          mount app.to_rack, at: to
         end
       end
     end