Update gh unit test workflow in AtoM to use ubuntu 24.04

[anvit]

GitHub is discontinuing Ubuntu 20.04, so we would need to update the unit test workflow to use 24.04.

PHP 8.x also breaks unit tests, since they're dependent on DebugPDO which has not been updated for PHP 8.0. The themselves tests also need updates since they contain a lot of PHP 8.x related deprecation warnings.

[erictapen]

I'm running into this problem as well, when trying to package AtoM for NixOS/Nixpkgs, which only has PHP 8. Having running unit tests always gives us some more assurance that the package is working as desired.

See my current efforts here: https://github.com/erictapen/nixos-atom