Removed support for Qt 5

[radarhere]

Pillow/src/PIL/ImageQt.py

Line 155 in 4b258be

elif im.mode == "I;16" and hasattr(qt_format, "Format_Grayscale16"): # Qt 5.13+

Pillow/Tests/test_imageqt.py

Lines 46 to 48 in 4b258be

	qt_format = ImageQt.QImage.Format if ImageQt.qt_version == "6" else ImageQt.QImage
	if hasattr(qt_format, "Format_Grayscale16"): # Qt 5.13+
	modes.append("I;16")

Support for Qt 5 was removed in Pillow 10.0.0, so this code can be simplified.