Don't set QT_MAC_WANTS_LAYER on modern macOS

[aronson]

When you boot up FlashGBX on modern macOS you get this warning in the console

qt.qpa.drawing: Layer-backing is always enabled.  QT_MAC_WANTS_LAYER/_q_mac_wantsLayer has no effect.

I don't like seeing it, so I researched. It was fixed in Big Sur. I added a guard to the environment variable set to only set it on platforms where it'll potentially have an effect.

[aronson]

Just saw this issue #6 and I see I missed the version by one major version. It was fixed in macOS 12. not 11, sorry for the confusion. Bumped up the version.