feat(milestones): add promote and burndown events endpoints

Zubeenrequested to merge
gitlab-community/gitlab-org/api/client-go:feat/milestones-promote-burndown-events into main

Adds the missing project milestone endpoints:

  • MilestonesService.PromoteMilestone -> POST /projects/:id/milestones/:milestone_id/promote
  • MilestonesService.GetMilestoneBurndownChartEvents -> GET /projects/:id/milestones/:milestone_id/burndown_events

Closes #2312 (closed)

Merge request reports