Thanks @indianer3c , I found, CDN for CKEditor in one of the files namely headitem.tpl at /source/Application/views/admin/tpl/headitem.tpl.
[{block name="admin_ckeditor"}]
<script type="text/javascript" src="https://cdn.ckeditor.com/4.22.1/standard/ckeditor.js"></script>
[{/block}]
by changing it to Long Term Support version , it requires us to buy the access. where as other versions still showing the alert to upgrade the version.