Migrates legacy dropdown in create_epic_form

What does this MR do and why?

This MR migrates legacy GlDropdown to GlListbox according to OKR

Resolves #418056 (closed)

Screenshots or screen recordings

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UI	Description
	Before
	After

How to set up and validate locally

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

1. Go to a group
2. Epics -> Create epic -> open created epic -> Click add in Child issues and epics
3. Verify dropdown with. groups

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.