Add Qt 6 support to CMake project

[vic-elgato]

This changes the following things

• increases required minimum cmake to 3.16
• modernizes cmake to set target scoped configurations
• removes Qt4 compatibility in cmake as it not mentioned in the readme anymore
• replaces QtKeychain find_packages with a FetchContent directly from github

When configuring the project it is necessary now to provide the CMAKE_PREFIX_PATH to the Qt version that should be used. That is the recommended way from Qt as documented at Getting started with cmake:

For find_package to be successful, CMake must find the Qt installation. There are different ways you can tell CMake about Qt, but the most common and recommended approach is to set the CMake cache variable CMAKE_PREFIX_PATH to include the Qt 6 installation prefix. Note that Qt Creator will handle this transparently for you.