Increase PHPStan level to 5
What does this MR do?
- Increase PHPStan level to 5
- Fix many issues preventing that
- Mute many other issues that we can't or won't fix
- Brush up Xhr/Xhr and remove
- please review each commit individually and refer to the commit messages!
How confident are you it won't break things if deployed?
I'll fix what's broken by it.
Links to related issues
How to test
- please review each commit individually and carefully
- no test since nothing should change
- no unrelated changes
- asked someone for a code review
- set a "for:" label to indicate who will be affected by this change
- use "state:" labels to track this MR's state until it was beta tested
- added an entry to CHANGELOG.md
- => no release notes
- no beta testing