Upgrade to Drupal 9 minimum PHP 7.3+

Getting the following on a Drupal 9 platform composer install:

drupal/core[9.0.10, ..., 9.0.x-dev] require php ^7.3 -> your php version (7.2.24) does not satisfy that requirement

Attempted to set the Debian PHP version from 7.2 to 7.3 in ansible/playbook.yml to no avail. It seems the max default available to Debian 9 is 7.2.

Proposed Resolution

Upgrade to Debian 10, as seen in https://docs.aegirproject.org/quick-start/.