Summary:
Pull Request resolved: https://github.com/facebook/folly/pull/1714

windows-2016 will be removed in March, and now that we've updated
Boost, we can upgrade to windows-2019.

Reviewed By: genevievehelsel

Differential Revision: D34119171

fbshipit-source-id: caad432bebf6b8282844bf79f82b1fbc34f2df9e