Cross-repository milestones [GA]

### Value Prop

Milestones are a key component of planning and tracking work on GitHub, allowing you to organise issues in a time-based group. Today milestones only work against the repository they are created in, which limits how useful they can be in projects which span across multiple repositories.

### Expected Outcome

With this release, milestones can now include issues from multiple repositories, making them much more useful for grouping and tracking issues—for example, when communicating a release, marking a point in time, or targeting a specific goal.