Port to Qt 6

[jmbowman]

The open source version of Qt 5 has been unsupported upstream for more than 3 years, and there are some other issues that can realistically only be resolved by upgrading to Qt 6:

• macOS builds on Ventura and higher are failing on a bug still present in the last open source Qt 5 release, which makes it hard to create a native Apple Silicon release in GitHub Actions. I think I can do it on a local machine, but that's the kind of release logistics hassle that's prevented me from making time to do a new release in almost 12 years...
• On August 31st, the required target SDK level for Android apps in Google play will be bumped to 34, which requires new enough versions of Gradle and the Gradle Android Plugin to run afoul of deprecated feature usage in Qt 5.15.

Unfortunately, QMacToolbar was also removed in Qt 6 along with other changes, so it won't be a trivial upgrade. But there's a native macOS toolbar class in https://github.com/nedrysoft/MacHelper which may prove to be a viable substitute, and the upgrade really needs to happen for the app to continue working on future OS versions. So this will be one of the top priorities after the long-overdue PortaBase 2.3 release.