Filter by any open milestone

New dynamic milestone filter that returns all issues that are assigned a milestone that is open. Call this Any Open. This would apply to filtering milestones in issues/mr lists, as well as the issue board config.

So for milestone filters we have currently (https://docs.gitlab.com/ee/user/project/milestones/#special-milestone-filters):

• 11.5 (An actual milestone)
• None
• Upcoming
• Started
• Any

This would be a new one.