Fix Throwable::getCode() method return type

[akondas]

Fixes: phpstan/phpstan#5664

[orklah]

This is not accurate. PDOException will return a string to getCode(), which means

try{
    throwsPDOException();
}
catch(Exception $e){
   var_dump($e->getCode());
}

will display a string for Exception::getCode()

[akondas]

Oh, I wasn't aware of that ... this should not be possible since you are excepting \Exception, thanks PHP

[orklah]

Well, the doc is actually incorrect, getCode returns int|string. However, in 99% of usages, it will return int, so I guess it was decided not to confuse people reading the doc. But yeah, this is something to consider when tweaking with static analysis tools