Show child issues on a board when filtering by an epic

What does this MR do and why?

This MR fixes the issue in #358416 (closed) where issue boards don't show all the issues in an epic. Right now it only shows direct children. This MR allows it to show all children including those in subepics.

References

Screenshots or screen recordings

Screen_Recording_2025-07-22_at_12.59.48

Before	After

How to set up and validate locally

1. Create an epic
2. Create an issue board that filters issues based on that epic
3. add an issue to the epic
4. add an epic to the epic
5. add an issue to the nested epic
6. check the board. Both issues should appear

MR acceptance checklist

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