Increment PHPStan to strictness level 2

[richardm-stripe]

PHPStan has "rule levels" https://phpstan.org/user-guide/rule-levels that determine which sorts of type errors they detect. I was surprised to discover we were on rule level 1. This PR adds type fixes and error suppressions to get us to rule level 2.

Almost everything is docstring-only changes, with the exception of

Fix unsafe static:: references. PHPStan doesn't like static:: references to private methods. I'm not 100% sure why but I think it's because

<?php
class Foo {
    private static function abc(){
        return "hello";
    }
    public static function xyz() {
        return static::abc();
    }
}
class Bar extends Foo {
    private static function abc() {
        return "world";
    }
}
print Bar::xyz();

will result in an error like

Fatal error: Uncaught Error: Call to private method Bar::abc() from scope Foo 

In any case, doing self:: instead makes it so that the private method defined in the parent class is always called, and prevents subclasses from re-implementing it, which I think is what is intended, anyway.