Fix issues preventing library from building with Qt 6

[Cuperino]

Changes:

1. CMake instructions now handle both Qt 6 and Qt 5
2. Pass correct QString-like type to setWindowTitle to fix build error
3. Remove qAsConst optimization to address deprecation warning
4. Increase CMake minimum version to address old CMake versions deprecation warning
5. Perform the install step after the Framework target property is set and add Framework indicator to the install call to address warning regarding possible incorrect path warning
6. Add CMakeLists.txt.user and build directories to .gitignore
7. Update GitHub workflow by adding Qt 6 and bumping macOS version

[dantti]

Also while you are at it, use macos-latest (& friends), the create build dir can be removed, and for Qt5.15 macos you can fix it by forcing only x86_64 builds, I fixed a few other kdab repos due new macos-latest being AppleSilicon now

[dantti]

see https://github.com/KDAB/GammaRay/blob/master/.github/workflows/build.yml you need to pass it to cmake

[dantti]

LGTM