v6.4.x to v7.0.0 Upgrade documentation

The v6.4.x to v7.0.0 upgrade documentation does not mention disabling older PHP versions in Apache. Including this step would make the documentation a little more concise.

You can check which PHP version Apache is currently using with:

`ls -l /etc/apache2/mods-enabled/ | grep php`

Then disable the old version with:

`sudo a2dismod phpX.X`