meta: split CHANGELOG into two files

The unfortunate has happened, our CHANGELOG is now over 1 MB and cannot
be viewed on github. This commit breaks the CHANGELOG into two files
so that we can continue to show our changes rendered in the github UI.

Closes: #5533

PR-URL: #6337
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Roman Reiss <me@silverwind.io>

Author: Myles Borins