When using an old PHP version (PHP < 7.1) display permanent message inviting users to upgrade PHP

[mattab]

In Matomo 4, we will require at least PHP 7+ (in #13611 )

And PHP 5.6 and PHP 7.0 are already out of the PHP support cycle and therefore all users should upgrade PHP to keep their Matomo data safe.

To prepare users for this, we want to display a permanent notification which will be visible and invite users to ugprade PHP (and warn them that Matomo will soon not work for them).

A similar code was done previously in #8193 #9144 #8156