Addition of Qt6 to LCG nightly builds.

This MR address https://sft.its.cern.ch/jira/browse/SPI-2325. The purpose of this MR is to add support for Qt 6.4.3 to the stack, which will be included alongside Qt 5. However, Qt 6 has been excluded ("blacklisted") from the LCG "views". Exclude Qt6 builds for Linux with clang <=12.

As part of this MR, the installation process has been updated to check for the presence of several Qt 6 libraries, including libQt6Core.so, libQt6Gui.so, libQt6Widgets.so, libQt6OpenGL.so, libQt6PrintSupport.so, libQt63DCore.so, libQt63DExtras.so, and libQt63DRender.so, using the check_qt6_libraries.sh script.