wix: bump the minimum Windows required to 8.1 PyOxidizer binaries are built with py 3.9.6, so not even Windows 7 is supported. Unfortunately, there don't appear to be more recent values for newer versions of Windows, but at least Windows 8.1 is still in extended support for another 9 months or so. Differential Revision: https://phab.mercurial-scm.org/D12596