Updated default php-fpm version to 7.3 for magento2 environments

[davidalger]

Magento 2.3.3 added support for PHP 7.3 and it is now enabled by default for magento2 environments.

If older versions of PHP 7.2 is still needed for your project, please ensure PHP_VERSION=7.2 is placed in the project's .env file to ensure that version is used now and into the future.

Warden 0.2.0 and later will begin including version variables such as this one in the .env file when written by env-init command to ensure future updates to default versions don't alter existing project service major versions.

[davidalger]

This was implemented in commits a6d9a1d and 2ba6f41.