Deprecate the `id_of` argument to `validators.create` and the `ID_OF` attribute on `Validator`s

These two are redundant with the now-anyways-needed Specification objects from referencing.

Either as part of this or some time in the future there will be a way of specifying which Specification should be associated with a new Validator, but regardless, these two objects are insufficient already today for encapsulating all the information that's needed to know how to deal with schemas for a dialect.

(#1061 is somewhat tangentially related to this).

Also somewhat related is expanding support for dialect <-> vocabulary interaction. The specification is still quite immature here and may change in the future, so there's a slight worry in creating an API that matches it too closely, but this ticket may benefit from "abandoning" jsonschema.validators.* in favor of new APIs that create dialects...

Start by tracing the validators.create API and the Validator.ID_OF attribute, then read how Specification objects from referencing are used. Review the relationship to issue #1061 and the dialect–vocabulary concerns described here. Done means the redundant APIs have a decided deprecation or replacement path that accounts for associating Specifications with new Validators.