Truncate the changelog pre-80 to a separate file.

Summary:
Splitting the CHANGELOG as it's getting too big to handle.

Changelog:
[Internal] [Changed] -

bypass-github-export-checks

Reviewed By: cipolleschi

Differential Revision: D77025333

fbshipit-source-id: cf2d54f2096f9c8c6a3fadb354d0c3065370440d

Author: cortinico