Update OS X build-from-source doc to using Qt6 by default

[Gudauu]

Reason for this PR:

• The current OS X build dependency specified in the Homebrew formulae defaults to Qt6 instead of Qt5. Therefore, Qt6 should be used as the default in CMake commands and docs.

What changed:

• In CMakeLists.txt, change WITH_QT6 to ON.
• Update the OS X build-from-source docs accordingly.

Tests:

• Tested on macOS 14.5; the build succeeded without problem.

Behaviour before this change:

• Followed docs precisely, build failed at the CMake stage with error: “Could not find a package configuration file provided by Qt5Widgets.”

[hluk]

This changes using Qt 6 by default everywhere and would break a couple of builds for Linux distros.

Would it be possible to make this automatic? Specifically:

• By default if the option is not overridden, pick Qt 6 preferably and if not found fallback to Qt 5.
• If set to ON by user, require Qt 6.
• If set to OFF by user, require Qt 5.