Use MilestoneFilter to sort out expiration

Why is this change being made?

Adopting #6185 (closed)

Now we try to treat expired milestones as closed.

MilestoneFilter will group the milestones into a hash:

• 'active': for any milestones which are not expired yet
• 'closed': for any milestones which are expired
• 'undefined': for any milestones which do not have a due date

We keep the API part because it's fine when we're looking for non-expired milestones in the pool of active milestones; and looking for expired milestones in the pool of closed milestones.

We can as well change it to look for all the milestones if needed.

• Direction: https://6185-adopt-expired-milestones.about.gitlab-review.app/direction/index.html
• Maturity: https://6185-adopt-expired-milestones.about.gitlab-review.app/direction/maturity/index.html
• Kickoff: https://6185-adopt-expired-milestones.about.gitlab-review.app/direction/kickoff/index.html
• Releases: https://6185-adopt-expired-milestones.about.gitlab-review.app/releases/index.html

Closes #6185 (closed)