Redirect users to SLO at the IdP after logging them out of Mastodon. (#24020)

app/controllers/application_controller.rb

@@ -61,8 +61,12 @@ class ApplicationController < ActionController::Base
   end
 
   def after_sign_out_path_for(_resource_or_scope)
+    if ENV['OMNIAUTH_ONLY'] == 'true' && ENV['OIDC_ENABLED'] == 'true'
+      '/auth/auth/openid_connect/logout'
+    else
       new_user_session_path
     end
+  end
 
   protected