Improve debug message and avoid warning due to escape format

src/Controller/LoginController.php

@@ -2,7 +2,7 @@
 
 namespace Drupal\ldap_sso\Controller;
 
-use Drupal\Component\Utility\SafeMarkup;
+use Drupal\Component\Utility\Html;
 use Drupal\Core\Access\AccessResult;
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Core\Url;
@@ -200,11 +200,11 @@ class LoginController extends ControllerBase implements LdapSsoInterface {
    */
   private function validateUser($remote_user) {
     if ($this->detailedWatchdogLog) {
-      \Drupal::logger('ldap_sso')->debug('Checking for invalid user.');
+      \Drupal::logger('ldap_sso')->debug('Starting validation for SSO user.');
     }
 
     $validator = new LoginValidator();
-    $validator->processSsoLogin(SafeMarkup::checkPlain($remote_user));
+    $validator->processSsoLogin(Html::escape($remote_user));
 
     if ($this->detailedWatchdogLog) {
       \Drupal::logger('ldap_sso')->debug('Remote user has local uid @uid',