Update epic illustrations

What does this MR do and why?

• Replace epics/roadmap illustrations with empty-milestone-md
• Replace epics/list illustrations with empty-epic-md

Outdated illustrations will be deleted here -> gitlab-svgs!991 (merged)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Go to http://gdk.test:3000/groups/awesome-test-group/-/epics, where awesome-test-group is your new group

2. Go to http://gdk.test:3000/groups/gitlab-org/-/epics/3

3. Add any child issue or create one

4. Open Roadmap view in the Child issues and epics section

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.

#430244 (closed)