qtbase: Fix dark mode detection on old Windows 10

Windows 10 builds before 1809 didn't support dark mode. Because Qt 6
doesn't support Windows 10 builds before 1809 they removed the check for
new enough Windows 10. This patch re-adds the check so that Gpg4win
doesn't attempt to use dark mode on old Windows 10.

• GnuPG-bug-id: T8181