fix clear dag run openapi spec responses by adding additional return type

[aliotta]

This Pr fixes the openapi documentation for the clearDagRun api. The api in question has two returns types depending on whether the dry_run param is true or false. Whilst it has no affect on the swagger ui at this time documenting these other return types helps when using code generation and will help us when openapi starts using the multiple return types in its ui.

[bbovenzi]

In airflow/www could you run yarn run generate-api-types. That should help with some static check failures. We may need to change some API tests too