Support Promote Issue to Epic within Change type modal

What does this MR do and why?

This Merge Request implements the ability to "promote" a Work Item Issue to an Epic, including the necessary namespace change to move the issue to the parent group.

To address this, a "promote" action is introduced to convert an issue into an epic while changing its location to the parent group.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• This issue #503276
• Main issue #385131 (closed)
• The MR thread !166879 (comment 2199754119)

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

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

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 FF work_items_beta

Steps to verify:

1. Login with any user
2. Go to Project > Issues
3. Open or create an Issue
4. In the kebab menu on top-right, find the Change type action
5. Click on the Change type action to open a modal with related options
6. Select Epic (Promote to group)
7. on Change type button
8. Verify if an Epic is created and redirected to
9. The original issue is closed

Related to #503276