[9.x] Adds not_starts_with validation

[mahmoudmohamedramadan]

This PR adds the ability to check if the string does not start with the given values, and I think this validation is important and makes the validation easier and we will not resort to using something else such as Regex.

$request->validate([
    'name' => ['required', 'not_starts_with:mahmoud'],
]);

@taylorotwell The PR sent to 9.x branch as you told :)