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 <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Roman Reiss <[email protected]>

Author: Myles Borins