View a burndown chart in the Iteration report
## Job Story
> When viewing an Iteration report, I want to see if my team is on track to complete their work for a given Iteration, so that I can surface trade-offs earlier on in the given timebox.
## Proposal
Include burndown charts in the iteration report.

UX: https://gitlab.com/groups/gitlab-org/-/epics/2422#group-by-ux
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