Move Authenticates2FAForAdminMode concern

This moves the file to app/controllers/concerns because
admin/concerns isn't a top-level autoload path so we need the
Concern:: namespace if we want it there

Since we also use this in
app/controllers/omniauth_callbacks_controller.rb, I think it'
better to just move instead of changing the namespace

config/initializers_before_autoloader/002_zeitwerk.rb

@@ -15,6 +15,7 @@ Rails.autoloaders.each do |autoloader|
   autoloader.ignore(Rails.root.join('lib/gitlab/graphql/docs'))
 
   autoloader.inflector.inflect(
+    'authenticates_2fa_for_admin_mode' => 'Authenticates2FAForAdminMode',
     'api' => 'API',
     'api_guard' => 'APIGuard',
     'group_api_compatibility' => 'GroupAPICompatibility',