Update the Gitlab Topic to use Group Milestones instead of Project Milestones
The current Gitlab Topic uses Project Milestones instead of Group Milestones.
But most of the Milestones in Gitlab are defined at a Group Level.
Update the Topic to use Group Milestones instead of Project Milestones. If we see that Project Milestones are also required, we'll add them with a follow-up issue, but at the moment I am leaving them out in order to not have unused options in Meltano UI.
Edited by Yannis Roussos