docs/about: Update the support statement for Windows

docs/about: Update the support statement for Windows

Our support statement for Windows currently talks about "Vista / Server
2008" - which is related to the API of Windows, and this is not easy
to understand for the non-technical users. Additionally, glib sets the
_WIN32_WINNT macro to 0x0601 already, which indicates the Windows 7 API,
so QEMU effectively depends on the Windows 7 API, too.

Thus let's bump the _WIN32_WINNT setting in QEMU to the same level as
glib uses and adjust our support statement in the documentation to
something similar that we're using for Linux and the *BSD systems
(i.e. only the two most recent versions), which should hopefully be
easier to understand for the users now.

And since we're nowadays also compile-testing QEMU with MSYS2 on Windows
itself, I think we could mention this build environment here, too.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/880
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <20220513063958.1181443-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>