[9.x] Add notFound helper to Http Client response

[erikgaal]

This PR adds some syntactic sugar to the Http Client Response class.

In a project we had to use the following code to check if a response was 404 Not Found

$response = Http::get('https://laravel.com');

if ($response->status() === 404) {
    doSomething();
}

You could still replace the 404 literal with a constant Symfony\Component\HttpFoundation\Response::HTTP_NOT_FOUND, but that adds a unnecessary dependency there.

With this PR you can do the following, which is a bit cleaner.

$response = Http::get('https://laravel.com');

if ($response->notFound()) {
    doSomething();
}