Switch the default test environment to PHP 8.4 and MySQL 8.4

Problem/Motivation

PHP 8.4 has been out for nearly half a year now, so I think we could switch the default test environment to use it.

Steps to reproduce

https://packagist.org/php-statistics

Proposed resolution

Switch default pipeline to PHP 8.4 and MySQL to 8.4 LTS as 8.0 is outdated https://dev.mysql.com/doc/refman/8.4/en/mysql-releases.html

Remaining tasks

agree and commit

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet