Revert milestone filter behavior for "any" (CE-backport)
CE-backport of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15014
What does this MR do?
Changes meaning of milestone=any
back to require a milestone.
Addresses regression https://gitlab.com/gitlab-org/gitlab-ce/issues/65676 by reverting the changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14586.