Bump to PHPStan v2 + ensure PHP 8.4 compatibility

src/Assertions/throws.php

@@ -4,7 +4,7 @@ namespace Trainline\Assertions;
 use Trainline\Exceptions\AssertionThrownException;
 use Trainline\Exceptions\AssertionThrowsException;
 
-function throws(callable $callback, string $expect=null) : \Throwable {
+function throws(callable $callback, ?string $expect=null) : \Throwable {
 	try {
 		$callback();
 	}