PHPStan - enable and fix

I got up to level 5 without having to add too many ignores due to external code. The 'next major' phpstan job is failing due to changes introduced in Drupal 11, but the tick on the module page doesn't take the next major jobs into account so I'm happy to leave those in a warning state, especially since the phpunit next major job entered a warning state while I was working on this. Drupal 11 is a bit of a moving feast.

Automatically closed - issue fixed for 2 weeks with no activity.