rector pass no 2: PHP modernizations

This modernizes all code as of PHP 8.1.

I did not apply the rule that would promote properties everywhere, because they would all end up in a single line. That is much less readable than before, so it needs a formatter rule to counteract it (which cs fixer does not have, unless we add someone's custom rule set)

Another point: obviously, this basically touches the entire project and will likely cause light merge conflicts everywhere, though they should hopefully not be that horrible to resolve