[8.x] Add prohibited validation rule

[PhiloNL]

In addition to the recently added prohibited_if and prohibited_unless validation rules (#36516), I think a generic prohibited validation rule may be useful.

Use case
In my specific use case, the prohibited validation rule is useful when building API's while working with immutable attributes, for example:

// Create a license key
$this->postJson(route('api.license'),['name' => 'foobar', 'key' => 'my-key']));

// Update given license key
$this->patchJson(route('api.license', '123-456'),['name' => 'hello-world', 'key' => 'random-key']));

The license key is immutable and for that reason it's not part of the validation rules, so the API responds with a successful status given it just ignores the key attribute:

// PUT /api/licenses/123-456
// {"name":"hello-world", "key":"random-key"}

$validated = $request->validate([
    'name' => 'required|max:255',
]);

$license->update($validated);

// Response: OK

When working with API's this might be confusing. This might give the impression that the request was successful and both the name and key have been updated while in reality only the name has been changed. The prohibited validation rule will throw a validation error if a prohibited attribute is submitted.

// PUT /api/licenses/123-456
// {"name":"hello-world", "key":"random-key"}

$validated = $request->validate([
    'name' => 'required|max:255',
    'key' => 'prohibited',
]);

// Response: 422
// The key field is prohibited