set default auth_mode for smtp mail driver (#5293)

Co-authored-by: KP <pohl@new-data-services.de>

config/mail.php

@@ -42,6 +42,7 @@ return [
             'username' => env('MAIL_USERNAME'),
             'password' => env('MAIL_PASSWORD'),
             'timeout' => null,
+            'auth_mode' => null,
         ],
 
         'ses' => [