Review toupper use [QWindowsKeyMapper]

This is a spin off from QTBUG-109235, please see there for more info.

I have a hard time deciding whether the toupper() call here:

qtbase/src/plugins/platforms/windows/qwindowskeymapper.cpp:        a = toupper(a);

is genuinely supposed to map i to İ} in the Türkiye locale, or whether it shouldn't still be {{I. If it is, then the code is wrong, because touppper('i') will likely return 'i', then, since İ isn't representable. So the code is wrong, either way. The question is what the fix should be.