PHPStan level 7

[shmax]

We have to get PHPStan up to at least level 8 if we want the dynamic find stuff to activate.

While I was at it, I cleaned up a lot of jank around Inflector and date_to_str.

[shmax]

@ipundit there are still about 110 stanning errors left at this level, and my eyes are starting to crust over from staring at a screen non-stop for two weeks, so I sort of need a break. If you have any energy left and feel like helping out, feel free to jump onto this branch and start knocking out more of those errors. 🙏