Mark MlFlushJobRequest as deprecated with future version notice (#3726)

Mark MlFlushJobRequest as deprecated with a future version notice.

Superseeds #3695 to comply with the validation workflows.

Author: valeriy42

output/schema/schema.json

@@ -35,6 +35,7 @@ import { DateTime } from '@_types/Time'
  * @availability stack since=5.4.0 stability=stable
  * @availability serverless stability=stable visibility=public
  * @cluster_privileges manage_ml
+ * @deprecated 9.1.0 Forcing any buffered data to be processed is deprecated, in a future major version a datafeed will be required.
  * @doc_tag ml anomaly
  * @doc_id ml-flush-job
  */