Switch the default test environment to PHP 8.4

Isn't it a bit too early for that? Still a lot of contrib modules still don't work well on PHP 8.3

solomon.yifru → made their first commit to this issue’s fork.

@danrod this is for core tests pipelines, we test on both PHP 8.4 and 8.3 every day, it has zero effect on contrib module test pipelines which are in the separate 'gitlab_templates' project.

I wonder would it make sense to switch to PHPUnit 11 for 8.4 too? And let 8.3 and below onto PHPUnit 10? https://stitcher.io/blog/php-version-stats-january-2025