Allow to promote milestone from the API

There is a feature to promote project milestone to a group milestone on GitLab UI which is missing from the API.

There should be an endpoint to do the same from the API.

/cc @victorwu

changed the description

changed milestone to %12.0

Let's do this to make our API as close as possible to Web UI.

added devopsplan label

added [deprecated] Accepting merge requests label

changed milestone to %Backlog

Is this request still eligible, as I would like to work on this ?

cc @victorwu

Thank you @nermin.vehabovic90 ! Yes it is. I have assigned you to the issue. Feel free to let us know if you have any questions.

assigned to @nermin.vehabovic90

Thanks for working on this @nermin.vehabovic90! We've removed the ~"Accepting merge requests" label to avoid having multiple people working on the same issue.

removed [deprecated] Accepting merge requests label

mentioned in merge request !25203 (merged)

mentioned in issue #57657 (closed)

closed via merge request !25203 (merged)

mentioned in commit 3b05d481

changed milestone to %11.9

added release post item label

removed release post item label