New cop to warn about `existing_card_id` usage #62

In d525445, we updated the `add_offense` method to let it work with older ruby versions. Everything worked fine until we started passing other params to the method, such as the severity, which doesn't work in Ruby 2.7 because the behavior is incorrect. Our way of calling the super method was introduced in Ruby 2.7, so we need to start using the new way from the correct version.