Create edge CI for PHP8
PHP 8.0.0 comes out Nov 26, 2020.
Create an appropriate set of edge test configurations for PHP8.
It appears that Drupal8 and Drupal 9.0 are not compatible with PHP 8, but that Drupal 9.1+ will be (https://www.drupal.org/node/3180764?utm_source=drupal-newsletter&utm_medium=email&utm_campaign=drupal-newsletter-20201119).
WordPress 5.6 is likely to be compatible (https://make.wordpress.org/core/2020/10/06/call-for-testing-php-8-0/)
Joomla aims to be compatible with minor PHP versions on day of release or within a few weeks but not for a major upgrade.
I'm not familiar with BackDrop's PHP8's compatibility efforts.
So I would start with WordPress and Drupal9.