Don't generate unusable project files

Even if the user selects "Target Qt Version"="Qt 5.15", Qt Design Studio automatically generates CMake files for Qt 6.

I understand that the CMake integration will not support Qt 5.15, so projects that target Qt 5.15 should not get any auto-generated CMake files by default, as these hinder integration into Qt Creator. Users who later upgrade to Qt 6 can manually export those files when the time is right.

Use-case

• Qt 5.15 is maintained till May 2025: https://www.qt.io/blog/qt-5.15-extended-support-for-subscription-license-holders
• Some embedded targets (e.g. i.MX7) are not supported by Qt 6, so some developers are still stuck on Qt 5.15