Allow group milestones to be used in issues

[zemzale]

Describe the feature or problem you'd like to solve

When creating issue using the glab issue create command and when selecting to add metadata in the interactive mode, group milestones are not showed.

▶ glab issue create
? Choose a template feature
? Title Something
? Description <Received>
? What's next? Add metadata
? Which metadata types to add? labels, milestones
? Select Labels To do
There are no active milestones in this project
? What's next? Submit

Propose a Solution

Fetch both project and group milestones when creating issues.

Additional context

These both APIs are separate, so we can probably fetch them concurrently and merge the data for that little bit of extra perfomance.
https://docs.gitlab.com/ee/api/milestones.html
https://docs.gitlab.com/ee/api/group_milestones.html

[clemsbot]

This issue has been automatically marked as stale because it has not had recent activity. We haven't had the time to address it yet, but we want to keep it open. This message is just a reminder for us to help triage issues.

[Hydragyrum]

It looks as though the issue is in the github.com/xanzy/go-gitlab package

[Hydragyrum]

Strike my last, it's implemented there

[clemsbot]

This issue has been automatically marked as stale because it has not had recent activity. We haven't had the time to address it yet, but we want to keep it open. This message is just a reminder for us to help triage issues.