chore: new PR changelog template (#5976)

Enables us to auto-generate the changelog from the list of PRs for a
modicum of summarizing/categorizing work on PR creation.

Does not (yet) allow external contributors to set category labels by
themselves as this creates issues with triggering one workflow from
another, it is not clear whether they should be allowed to create new
categories, and the reviewer/triage team likely is in a better position
to do the categorization anyway.