MDL-66581 auth: Do string concatenation for the password change URL

This reverts the change introduced by MDL-59298 where the password
change URL data is being assigned with a moodle_url object instead
of concatenating it.

lib/authlib.php

@@ -785,7 +785,7 @@ class auth_plugin_base {
         $USER = $user;
         if ($this->can_change_password() and $this->change_password_url()) {
             // We have some external url for password changing.
-            $data->link = $this->change_password_url();
+            $data->link .= $this->change_password_url();
         } else {
             // No way to change password, sorry.
             $data->link = '';