* Fix Windows CI build.

The Windows SDK versions present on our CI options (GitHub Actions
hosted runners, AppVeyor) have headers that cause compilation warnings
when included with MSVC operating in modern C modes (C11+).

This PR disables the compilation warning caused by the Windows SDK
headers, so we can get Windows feedback from CI. The warnings can be
re-enabled when the Windows SDK used by our CI is upgraded to a version
that doesn't trigger warnings.

* Switch CI to GitHub Actions.