API: iid filter changes for projects/:id/milestones

In order to unify the way we filter milestones in V4 API, we should rename iid filter to iids (keeping it for multiple resources fetching with array filter). For single resource fetching, we should use projects/:id/milestones/:milestone_id.

Obs.: V3 namespace should remain unchanged.

Related issues: #25183 (closed) #27180 (closed) #20070 (closed)