Migration to destroy/redirect dupe announcements

Removes a couple of duplicate announcements and sets up redirects for them.

There's a separate pull request to update router-data: https://github.gds/gds/router-data/pull/144

Story: https://www.agileplannerapp.com/boards/173808/cards/5667
we need to clean-up related `StatisticsAnnouncementDate` records here. might even want to consider adding `dependent: :destroy` to the `has_many` definition.