Fix PHPStan issues reported on CI/gitlab

MR created, note that this includes 🐛 Use of dynamic properties is deprecated in PHP 8.2+ Active , please credit users there as well, thanks

I created 2 branches so far:
- 3502957-fix-phpstan: this contains all phpstan fixes excluding DI, no BC breaking changes.
- 3502957-fix-phpstan-di: I started fixing some DI, but not all. I believe it breaks BC? pretty sure this won't happen anytime soon... Some of those are very easy fixes like \Drupal::messenger. Maybe a good way forward is to fix those easy ones and add inline phpstan exclusions @phpstan-ignore-next-line GlobalDrupalDependencyInjectionRule everywhere else. So we maintain BC, the CI will be green and new code has to be compliant.

I reviewed all of these phpstan changes and approve!

Message to the maintainers:

Please add credit to: recrit → recrit worked on the deprecation of dynamic properties fix which is part of the phpstan fixes included above.