Use burnup graphQL endpoint for milestone charts

What does this MR do?

Ref #6903 (closed)

1. Use burnup graphQL endpoint for burnup charts. Requires the burnup_charts feature flag
2. Add 'Completed' line to burnup chart
3. Add Weight values for burnup chart (previously the Weight button only worked for burndown)

Screenshots

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