Use Qt6 for macOS build

[rerrahkr]

#468

Older versions of Qt seem to have a problem with QSlider not rendering properly with the recent Monterey update (https://bugreports.qt.io/browse/QTBUG-98093).

This is a PR to see if changing the dev build in Appveyor to use Qt6 solves this problem.
In some cases, the GHA release build recipe may also need to be modified.

[OPNA2608]

Times out because AppVeyor's macOS image is on 10.15 Catalina for which Homebrew doesn't carry a prebuilt version of Qt6, so it tries to build Qt6 from source:

Sticking with Homebrew's Qt will have to involve an upgrade to a newer macOS image.

Alternatively, we could try using one of the pre-installed Qt6's instead:

[rerrahkr]

I missed the prebuild Qt. macOS jobs in GitHub Actions use Big Sur, so homebrew should work fine.

[OPNA2608]

Feel free to take some of my changes from here if you'd like, replaces the AppVeyor Homebrew usage in favour of the pre-installed Qt 6.4 and no JACK support (saves time where it would otherwise have to update some Homebrew stuff, build Catalina packages that got removed etc): master...OPNA2608:BambooTracker:appveyor_qt6_preinstalled

[rerrahkr]

Mergerd. Thanks for your help!