Use Solidus 4.4 Redirect extension points if available

This makes the `prepare_{frontend,backend}` methods which instantiate
the base controller and the admin base controller, requiring all of
ActionController::Base, unnecessary to call for newer Solidus versions.

Depends on solidusio/solidus#6051

lib/spree/auth/engine.rb

@@ -21,10 +21,19 @@ class Engine < Rails::Engine
         Spree::Auth::Config = Spree::AuthConfiguration.new
       end
 
-      config.to_prepare do
-        Spree::Auth::Engine.prepare_backend if SolidusSupport.backend_available?
-        Spree::Auth::Engine.prepare_frontend if SolidusSupport.frontend_available?
+      if Spree::Config.respond_to?(:unauthorized_redirect_handler_class)
+        Spree::Config.unauthorized_redirect_handler_class = "Spree::Auth::UnauthorizedCustomerAccessHandler"
+        if SolidusSupport.backend_available?
+          Spree::Backend::Config.unauthorized_redirect_handler_class = "Spree::Auth::UnauthorizedAdminAccessHandler"
+        end
+      else
+        config.to_prepare do
+          Spree::Auth::Engine.prepare_backend if SolidusSupport.backend_available?
+          Spree::Auth::Engine.prepare_frontend if SolidusSupport.frontend_available?
+        end
+      end
 
+      config.to_prepare do
         ApplicationController.include Spree::AuthenticationHelpers
       end