Better axis labels for burndown charts

What does this MR do?

1. Set min of 1 day on x-axis (as that is our bucket size)
2. Set min of 1 on the y-axis (prevents values like '200m')
3. Format dates in a more human-friendly way (matches rest of Milestone/Iteration date displays)
4. Remove avg/max from chart legend (not very relevant for burndown charts)

Fixes 2/3 of #213588 (closed)

Screenshots

See Burnup chart (on the right) for the differences

before	after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content