Upgrade to PHPUnit 12

[HypeMC]

Resolves #1051

[nicolas-grekas]

Let's bump to v12?

[HypeMC]

@nicolas-grekas PHPUnit 12 requires PHP 8.3, so I had to allow older versions for lower PHP versions.

[nicolas-grekas]

I did the bump to PHP >= 8.1 in #1067
Rebase needed for any follow up here :)

[HypeMC]

I did the bump to PHP >= 8.1 in #1067 Rebase needed for any follow up here :)

@nicolas-grekas Done

[nicolas-grekas]

I forced-pushed a rebase, then realized that while tests are green, they have things to fix (one risky, and N ones?)

[HypeMC]

@nicolas-grekas Oh, the N test are because of PHPUnit 12.5-dev, guess that's something new, I'll have to check it out.