Update milestone banner illustration

What does this MR do and why?

• Update milestone banner illustration
• Remove old illustration

Screenshots or screen recordings

Before	After

How to set up and validate locally

If you have a CE version:

1. Open any project
2. Go to Plan > Milestones
3. Create a milestone
4. See the banner

If you use EE version:

1. Open ee/app/views/shared/promotions/_promote_burndown_charts.html.haml file
2. Comment - if show_burndown_charts_promotion?(milestone) && show_callout?(callout_id) line
3. Open any milestone you have
4. See the banner

#430246 (closed)