Add PHP 7.2 and PHP 7.3 builds to all core modules

[chillu]

Overview

PHP 7.0 and PHP 7.1 are out of support pretty soon. We technically support PHP 7.2 and PHP 7.3 in framework already, and test it there. We should ensure this compatibility across all core modules and their supported minor release branches. This includes SS 3.7.

We should aim to keep the number of builds the same, and remove unsupported PHP versions from the build.

Notes

• Adding it to non-core supported modules is out of scope, since that falls into the responsibility of the Creative Commoners team.
• Related: https://github.com/silverstripe/cwp-recipe-kitchen-sink/issues/22
• Related: https://github.com/silverstripe/silverstripe-framework/issues/8655

[chillu]

Thanks to our open source hero @robbieaverill this is the case now :)