Warning about deprecated AA_UseHighDpiPixmaps should be versioned

Applications that need to build with both Qt5 and Qt6 are getting a ton of warnings such as

warning: ‘Qt::AA_UseHighDpiPixmaps’ is deprecated: High-DPI pixmaps are always enabled. This attribute no longer has any effect. [-Wdeprecated-declarations]
   27 |     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);

Using the versioned deprecation macros would allow users to disable deprecation warnings for Qt >= 6, saving them from ifdefing their code

Assigning to "build system", since this applies to all enums, not just the HDPI ones