Return `HTTP 410 Gone` status code from `POST /runners` endpoint if registration tokens are disallowed

According to https://docs.gitlab.com/ee/architecture/blueprints/runner_tokens/#details, we need to make POST /api/v4/runners endpoint based on a registration token permanently return HTTP 410 Gone if allow_runner_registration_token setting disables registration tokens. A follow-up will do the same in response to the :enforce_create_runner_workflow feature flag.

A future v5 version of the API should return HTTP 404 Not Found.