Update testing configuration

This project currently has one automated test configuration for issue queue testing:
PHP 7.4 & MySQL 5.7, D9.3.0 tested on commit, issue testing default

There is nothing wrong with this.

I personally find it useful to have additional test configurations.

What I usually do is have three test configurations:

• One to test the module against the lowest-supported versions of core Drupal and PHP.
• One to test the module against the currently recommended version of core Drupal and using MySql 8.
• One to test the module against the development version of core Drupal and the most recent version of PHP, performed weekly so we can get early warning of any coming changes in core.

The reason for this is that I always want my modules to work with all supported versions of Drupal, and I also want to test things I know might be a problem - for example MySql 8 may cause problems because it has a lot API changes from the MySql 5 version that most people use. Likewise with PHP 8, which has a lot of API changes over PHP 7.

Right now, my preferred configuration is this:
PHP 7.3 & MySQL 5.7, D9.2.10 30 pass tested on commit, issue testing default
PHP 7.4 & MySQL 8, D9.3 30 pass tested on commit
PHP 8.0 & MySQL 5.7, D9.4 30 pass tested weekly

I triggered some tests in #3263221: Update composer.json → and as you can see, there are no test failures with MySql 8 or with PHP 8. That's good. I think the easiest way to keep it like that is to have some additional test configurations.