Update CHANGELOG formatting (#682)

The changelog now uses a style slightly more consistent with
https://keepachangelog.com so that:
1. It's more consistent with our other buildpacks (including the PHP CNB)
2. The changelog can be updated by the new "prepare release" shared workflow

As an added bonus, the new style includes GitHub compare URL links for each
version heading, making it quicker for users to see a full diff of changes between
each release.

GUS-W-14888899.