Build/Test Tools: Expand database testing to account for all supported versions and types.
This modifies the PHPUnit workflow to add MySQL version 8.0 to the strategy matrix. It also adds an additional strategy matrix to expand testing to include several supported MariaDB versions.
To prevent repeating code, the business logic of the PHPUnit testing has been moved to a new, callable workflow.
Because callable workflows target a specific branch, branches created in the future will continue to benefit from improvements made to the workflow in trunk without requiring backports. If a breaking change is required, older branches will need to be updated to target the commit SHA representing the final commit of the previous state.
Props johnbillion, pento, boonebgorges, netweb, nacin, desrosj.
Fixes #30462.