Disable building of Qt 6 using qmake

To avoid having to maintain 2 build systems for building Qt, we should disable the option of building Qt with qmake.

This implies the following:

• Disallowing use of configure using qmake
• Disabling qmake Coin CI configurations

The following are some newly discovered items regarding missing coverage that should be addressed:

• In order not to lose qmake example CI coverage, we need to implement instructions to build all qmake examples as standalone projects.
• Grafana test result collection
• Official documentation generation packages still use qmake

A separate task exists for removing the Qt build system files. Strictly speaking that is separate from disabling qmake Qt build support.
We have to be careful not to remove qmake support files that are required for building qmake applications (like mkspecs and certain .prf files).

Outcomes / Consequences:

• We can't use pro2cmake anymore to regenerate CMake projects. The CMake projects become the source of truth now.
• Large refactoring of CMake projects not easy anymore without pro2cmake
• Have to decide what to do with configure.json/configure.cmake files, do we still use configurejson2cmake, or do we switch the source of truth to the configure.cmake files