Description of the problem, including code/CLI snippet

This request is for supporting the ability to 'promote' a project milestone to a group one. This exists in the Gitlab API.

Specifically, https://docs.gitlab.com/ee/api/milestones.html#promote-project-milestone-to-a-group-milestone

Expected Behavior

project.milestones.promote(milestone_id)

Actual Behavior

Promotion not supported

Specifications

• python-gitlab version: latest
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): Any