Fix qtzlib support for Qt6, add CI tests

[cen1]

• add 3 docker images with qt5 and qt6 static builds for usage in CI
• add Linux and Windows CI for qtzlib option. Static Qt on Windows CI runners is built from source because no better option seems to be available. Using cache somewhat improves the runs other than the first.
• fix qtzlib for Qt6, now links to both BundledZLIB (static lib) and ZlibPrivate (headers)
• fail fast if Qt zlib module is not found to prevent accidental linking to system zlib
• fail fast if using qtzlib and shared options at the same time