Update epic deletion confirmation

What does this MR do and why?

When you delete an epic, the confirmation dialog warns of deleting all descendants, which is no longer true since Enhanced Epic Deletion Protection (#452189 - closed)

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

Type	Before	After
Legacy Epic
Objective
Epic work items

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Steps to verify on legacy Epics:

1. In a group where you're an owner (for example http://gdk.test:3000/groups/flightjs), create an epic.
2. On the epic, select Epic actions ({ellipsis_v}), then Delete epic.
3. Select Delete. On the confirmation dialog, select Delete epic.

Steps to verify on Objectives:

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. Login with any user
3. Go to any Project > Issues > List
4. Create an new Objective under New issue split button
5. Go to Objective detail page
6. Click on delete
7. Check if the confirmation message shows - Are you sure you want to delete the objective? This action cannot be reversed.
8. Add few child items
9. Click on delete
10. Check if the confirmation message shows - Delete this objective and release all child items?

Steps to verify Epic work items:

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 the feature flag Feature.enable(:namespace_level_work_items)
3. Login with any user
4. Go to any Group > Epics > List
5. Create an new Epic
6. Go to Epic detail page
7. Click on delete
8. Check if the confirmation message shows - Are you sure you want to delete the epic? This action cannot be reversed.
9. Add few child items
10. Click on delete
11. Check if the confirmation message shows - Delete this epic and release all child items?