Changelog: Document Bitbucket Server burndown chart support

Created by: mrnugget

This documents the changes we made as part of #6085. It documents the new feature ("Burndown chart support for Bitbucket Server") and also mentions that this leads to more requests being sent to Bitbucket Server instances and why. It also mentions that in the future we might get rid of the new requests, since we'll add webhook support.

Wording suggestions/changes: highly welcome! Feel free to edit etc.