Use config_for for omniauth enabled values (#35015)

app/controllers/application_controller.rb

@@ -98,7 +98,7 @@ class ApplicationController < ActionController::Base
   end
 
   def after_sign_out_path_for(_resource_or_scope)
-    if ENV['OMNIAUTH_ONLY'] == 'true' && ENV['OIDC_ENABLED'] == 'true'
+    if ENV['OMNIAUTH_ONLY'] == 'true' && Rails.configuration.x.omniauth.oidc_enabled?
       '/auth/auth/openid_connect/logout'
     else
       new_user_session_path