Officially discontinue the Windows build

There's discussion on the forums about [no longer offering official windows binaries](https://forums.foundationdb.org/t/looking-for-community-support-for-the-windows-build/2531), but I don't see an official "They're no longer being produced" message. With no official statement for this, there are community members starting to get confused (see #4980 and #4699).

So as the binaries are no longer being published, there should probably be official guidance as to that fact:

- [ ] The downloads page should either not list windows, or mention that windows support has been discontinued.
- [ ] We should retroactively add a release note to 6.3 to mention that windows support was discontinued.
- [ ] References to Windows in the documentation should be removed.

And as the stated goal in the forums post was to look for community support, we should start to consider how to enable community support for other OSs, but that should probably be a separate issue. (FreeBSD would be our other community-maintained OS port.)