…lastic#80905)

In elastic#80731 the infer trained model task was correctly set to have as
parent task the internal infer action task when called from there.
However, it was done by both setting `Request.setParentTaskId` and
using a `ParentTaskAssigningClient`. There is no need to use a
parent task client. Instead, to set the parent task on the request
we should use `setParentTask` instead of `setParentTaskId` which
effectively sets the target task for a `BaseTasksRequest`.

The confusion of `BaseTasksRequest` holding two fields both names
`parentTaskId` and having two methods both setting the parent task
id will be addressed in a separate PR.

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>