Upgrade to final PHP 5.6.40 version

Summary:
PHP 5.6.40 is the final version of PHP 5.

This change also updates keyservers, as SKS doesn't work anymore.

Reference: https://www.php.net/eol.php

Test Plan: Build image

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: Nasqueron Docker deployment squad

Tags: Docker images

Differential Revision: https://devcentral.nasqueron.org/D2524