Disable child items list on adding or reordering items in work items

What does this MR do and why?

As the mutations have some of the limitations to perform optimistic updates and cache write, disable child items list on adding or reordering items in work items. This will avoid impromptu errors on the frontend when actions are performed simultaneously.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Scenario	Screen recording
Creating and adding work item	Screen Recording 2024-08-02 at 4.36.52 PM.mov
Reordering a work item	Screen Recording 2024-08-02 at 4.39.06 PM.mov

How to set up and validate locally

Prerequisites:

1. Enable the OKRs feature as mentioned in the OKRs docs (e.g. ::Feature.enabled?(:okrs_mvc, Project.find(<PROJECT_ID>)) and the namespace must have Ultimate plan).
2. Enable Epic work items
   1. Feature.enable(:work_item_epics_rollout)
   2. Feature.enable(:work_item_epics)

Steps to verify:

1. Login with any user
2. Go to Project > Issues
3. Create an Objective or an Issue
4. Add few child items in the Child items widget
5. Notice the list while adding or creating the work items
6. Reorder item to check whether the list is getting disabled

The same can be tested on work item Epics Child items widget in the Group > Epics path.

Related to #475002 (closed)