Add "create work item" modal to legacy issues list

What does this MR do and why?

Add "create work item" modal to legacy issues list (#514577)

References

• #514577

Screenshots or screen recordings

Screen_Recording_2025-06-17_at_11.01.55_pm

How to set up and validate locally

1. Enable issues_list_create_modal feature flag
2. Go to the issues list page e.g. http://127.0.0.1:3000/groups/flightjs/-/issues
3. Test the "New item" modal

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 #514577