Can I upgrade CentOS 6 to 7?

[cPRex]

You could always create a new Destination server with MariaDB using the details here:

Customize Your Installation | cPanel & WHM Documentation

This document describes how to customize your cPanel & WHM installation.

docs.cpanel.net

but you'd have to reinstall the server to use that option.

 

[andrew.n]

or you can do MySQL 5.7 as well which is the closest to 5.6 and still supported