Move GraphQL burnup queries to new format

As part of #233707 (comment 413526459), we wanted to move the burnup query under a report field so that we can add extra information computed from the timebox events.

Steps needed:

• Create new report field for milestones and iterations (!45121 (merged))
• Change frontend to use this new field (!46946 (merged))
• Deprecate / remove the old burnupTimeSeries field (the one not under report). We could then remove ::Resolvers::TimeboxBurnupTimeSeriesResolver.