Milestone labels can be confusing => Renamed to Release scoping labels

We recently changed the Deliverable, Stretch, and Next Patch Release to be called Milestone labels (there were previously known as Prioriry labels): https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#milestone-labels-deliverable-stretch-next-patch-release.

I feel this change could be a little confusing since we already have a milestone "attribute". So, having both a milestone attribute and a milestone label will lead to ambiguous cases when you just refer to the "milestone".