Remove parent_id arg on BulkUpdate mutation

What does this MR do and why?

• This MR cleans up an argument originally introduced to the work items BulkUpdate mutation. This parent_id argument was switched in favour of full_path, which is now in use by the frontend.
• Note that the work items bulk update mutation is currently marked as experimental, allowing us to make this change

References

• #556058 (closed)

Screenshots or screen recordings

Not provided

How to set up and validate locally

• Verify that the work items bulk update functionality continues to behave as expected
  1. With the GDK running, navigate to the epics page for a group (e.g. http://localhost:3000/groups/flightjs/-/epics)
  2. Select any number of epics and apply a bulk update
  3. (optional) observe in the network tab that the mutation sends fullPath to backend, and not parentId
  4. Observe the epics were updated accordingly

MR acceptance checklist

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Related to #556058 (closed)