Migrate open milestones' due date

Our Redmine milestones have due dates, but we don't migrate them yet. This info is useful to display milestones in the expected order.

We don't have too many open milestones so it's OK to do it post-migration with GitLab'abracadabra.

While we're at it, for each milestone, we could manually set the "start date" to the scheduled release date of the preceding release: this allows filtering issues based on "Milestone = Started" and "Milestone = Upcoming".