Changes to Dynamic Workflow Trigger

removed the media type application/json for the response with the status 202

removed the media type application/json for the response with the status 400

the trigger_at request property format was generalized from date to date-time

added the media type text/plain for the response with the status 202

added the media type text/plain for the response with the status 400

removed the request property users/items/$slack/items/oneOf[subschema #4: Using incoming webhook]/properties

removed the request property users/items/$slack/items/oneOf[subschema #4: Using incoming webhook]/required

removed the request property users/items/$slack/items/oneOf[subschema #4: Using incoming webhook]/type