Use modal creation form for "New issue" from global nav

What does this MR do and why?

This MR updates the "New issue" dropdown item from the global nav to show the "create work item" modal (#517541 (closed))

Changelog: changed

References

• #517541 (closed)

Screenshots or screen recordings

Screen_Recording_2025-06-30_at_6.02.18_pm

How to set up and validate locally

1. Go to a project, then click "New issue" in the global nav
2. Go to a group, then click "New epic" in the global nav

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.

Related to #517541 (closed)