Announce deprecation - change in %upcoming filter logic

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Summary

In #429728 (closed) we will update the logic for %upcoming and %started board filter options. Since this is technically a breaking change, we need to announce this following this guide](https://docs.gitlab.com/ee/development/deprecation_guidelines/), so that we can make the change in %18.0

Upcoming is currently documented here: https://docs.gitlab.com/ee/user/project/milestones/#special-milestone-filters