Change MSW version checks to work for yet unreleased versions

wxGetWinVersion() will return wxWinVersion_Unknown for any MSW version
not recognized by the version of wxWidgets used when building the
application, so it makes sense to use a value higher than all currently
known ones for this constant, so that the checks comparing the result of
wxGetWinVersion() with any known version would still work correctly.

E.g. comparisons like wxGetWinVersion() >= wxWinVersion_Vista will now
continue to work for binaries built using the current wxWidgets even
under some future Windows 11 OS version, unlike before.

include/wx/msw/private.h

@@ -902,8 +902,6 @@ inline wxString wxGetFullModuleName()
 // for the other Windows versions 0 is currently returned
 enum wxWinVersion
 {
-    wxWinVersion_Unknown = 0,
-
     wxWinVersion_3 = 0x0300,
     wxWinVersion_NT3 = wxWinVersion_3,
 
@@ -930,7 +928,11 @@ enum wxWinVersion
     wxWinVersion_8 = 0x602,
     wxWinVersion_8_1 = 0x603,
 
-    wxWinVersion_10 = 0x1000
+    wxWinVersion_10 = 0x1000,
+
+    // Any version we can't recognize will be later than the last currently
+    // known one, so give it a value greater than any in the known range.
+    wxWinVersion_Unknown = 0x7fff
 };
 
 WXDLLIMPEXP_BASE wxWinVersion wxGetWinVersion();