ci: establish a CHANGELOG file

Establish a CHANGELOG file to be updated on each release.  This changelog only contains
changes being with `12.0.0-next.0`, prior release notes will continue to be available
in the Github Releases UI.  To generate this changelog, the body contents of each release
published to the Github Releases was extracted.

(cherry picked from commit 4a08ec0)

Author: josephperrott