Update test environment to PHPUnit 10

[clue]

PHPUnit 10 has been released (yesterday) 🎉 Time to update our test suite to take advantage of the latest PHPUnit version on supported PHP versions (PHP 8.1+). All preparations have already landed in previous PRs linked below, so this update was really smooth sailing.

Builds on top of #207, #194, #199 and others

[SimonFrings]

I can approve this pull request, but I think we should mention that we might loose future visibility of deprecation notices. I looked a bit further into this.

In the changelog for PHPUnit 10 it says the following:

Removed the convertDeprecationsToExceptions attribute on the XML configuration element as well as the feature it used to control

So it seems we can't use convertDeprecationsToExceptions anymore, but there is a new option called displayDetailsOnTestsThatTriggerDeprecations. Not sure if this will have the same effect tho.

One solution to this could be to add another phpunit.xml specifically for PHPUnit 9 (PHP >= 7.3), but this feels kind of dirty, what do you think?

[clue]

Agree that we'd want to keep this functionality, fortunately looks like this might be re-introduced in PHPUnit 10.1 again via sebastianbergmann/phpunit#5196

We can keep this open until the upstream ticket is resolved, looks like we'll need to use separate XML configurations for PHPUnit 10, PHPUnit 9 and older in either case to support the current range of PHP versions. Not a fan of this, but fine, if that's what it takes.

[clue]

As per the above discussion, I'll close this for now and we can look into this again some time later once PHPUnit 10.1+ is released. In either case, the test suite has been updated via #217 in the meantime anyway.