wix: bump the minimum Windows required to 8.1...
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
