Use constant since string/symbol was removed

For fun, autoload doesn't work in application.rb, so you have to either
do it in an config/initializers or require the file.

rails/rails@83b767ce
Rails issue: #25525

Extracted from ManageIQ#18076

config/application.rb

@@ -109,7 +109,8 @@ class Application < Rails::Application
 
     config.autoload_once_paths << Rails.root.join("lib", "vmdb", "console_methods.rb").to_s
 
-    config.middleware.use 'RequestStartedOnMiddleware'
+    require_relative '../lib/request_started_on_middleware'
+    config.middleware.use RequestStartedOnMiddleware
 
     # config.eager_load_paths accepts an array of paths from which Rails will eager load on boot if cache classes is enabled.
     # Defaults to every folder in the app directory of the application.