Convert milestone burndown chart into ECharts
Description
We currently use chartjs to render the milestone burndown chart found in https://gitlab.com/groups/gitlab-org/-/milestones/33. We need to change it to ECharts to
- Reduce dependency on multiple charting libraries
- Confirm charts to design pattern
Screenshot
Approximation of what it would look like in ECharts
Edited by Amelia Bauerly