One more PHP 8.1 deprecation fix (#374)

* PHP Deprecated:  Return type of MauticPlugin\CustomObjectsBundle\Entity\CustomFieldOption::offsetGet($offset) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed

* Compativility with PHP 7.4

Author: escopecz

Entity/CustomFieldOption.php

@@ -171,6 +171,10 @@ public function offsetExists($offset): bool
 
     /**
      * {@inheritdoc}
+     *
+     * @param mixed $offset
+     *
+     * @return mixed
      */
     public function offsetGet($offset)
     {