Update display_work_item_epic_issue_sidebar to work_item_epics

What does this MR do and why?

We are consolidating all FF related to GA of work item epics under a single feature flag. Update the display_work_item_epic_issue_sidebar to new flag work_item_epics.

⚠️ Note: This MR should be merged only after !157443 (merged) is merged.

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

No visual changes

How to set up and validate locally

Prerequisites:

1. The namespace must have Ultimate plan.
2. Enable the OKRs feature as mentioned in the OKRs docs (e.g. ::Feature.enabled?(:okrs_mvc, Project.find(<PROJECT_ID>)).
3. Enable Feature.enable(:namespace_work_items_level)
4. Enable Feature.enable(:work_item_epics)

Steps to verify:

1. Login with any user
2. Go to a Group/Project which has few Epics and Issues created under them
3. Goto an issue
4. Assign a Parent/Epic in the Epic widget present in the sidebar of Issue
5. Make sure the list is proper
6. Disable the flag Feature.disable(:work_item_epics)
7. Check the same flow

This also affects Project > Issue Boards. Click on any board item to open a sidebar and check if the Epic widget functionality works properly.

Related to #467686 (closed)