doc: Fix incorrect version list for Windows SDK
References to SDK 8.1a should have been cleaned up in
495ed0e, as only
version 10 and above are supported with Windows 10 and newer versions.
Reported-by: Thomas Munro
Discussion: https://postgr.es/m/CA+hUKG++qE9uVjQaTTD7oaC8a2T3h8K50=Eqyx9uUZvOHa__ww@mail.gmail.com