DEV: Make Ember CLI assets the default in production (#15843)

This can be disabled by setting `EMBER_CLI_PROD_ASSETS=0`, but this option will not be available for long. If your theme/plugin/site has issues under Ember CLI, please open a topic on https://meta.discourse.org

app/controllers/qunit_controller.rb

@@ -23,7 +23,7 @@ class QunitController < ApplicationController
 
     @is_proxied = is_ember_cli_proxy?
     @legacy_ember = if Rails.env.production?
-      ENV['EMBER_CLI_PROD_ASSETS'] != "1"
+      ENV['EMBER_CLI_PROD_ASSETS'] == "0"
     else
       !@is_proxied
     end