Use GitLab UI's GlEmptyState on Roadmap empty state

What does this MR do and why?

Migrate the existing component to use the empty-state component from GitLab UI

Issue: #300055 (closed)

Screenshots or screen recordings

before	after

How to set up and validate locally

Visit the Epics > Roadmap to view the updated screen.

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.

Related to #300055 (closed)