Adopt new `discriminated` decorator for discriminated unions

The old way of doing this with the `discriminator` decorator has been removed in https://github.com/microsoft/typespec/pull/6286.

Here is the design - https://github.com/microsoft/typespec/issues/2738

Prior to GA, TCGC will report an error. After GA, TCGC will add support for this decorator, so we will need to adopt it and re-enable the now failing Spector tests.