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.

[HypeMC]

@nicolas-grekas Should be good now, see https://github.com/symfony/flex/pull/1056/commits/4dc8127fd171072fe07b47107337f2bd212c8c39.

The main issue was:

No expectations were configured for the mock object for <class>. You should refactor your test code and use a test stub instead.

Also, using $this->any() resulted in the same error, eg:

$repository = $this->createMock(InstalledRepositoryInterface::class);
$repository->expects($this->any());