Provide a nightly build to test the use of C++ in limits of the C++17 standard

In the default target configurations, we rely on the default selection of the C++ standard set in the compiler toolchain. We basically omit using the "-std=c+ZZ" option. With this, a developer might use any features of a C+ standard that are whitelisted in the default settings in a given version of the underlying compiler toolchain, regardless of a specific standard, see https://gcc.gnu.org/onlinedocs/gcc/Standards.html and https://gcc.gnu.org/projects/cxx-status.html#cxx17

We have a considerable number of embedded customers which rely on a certain minimal C++ standard version expected in Qt because their toolchain just does not support newer C++ standards. For Qt 6.0 and higher until further notice, this minimal C++ standard is set to "C++17".

A test build is needed to ensure that Qt accidentally does not include C++ features that would not be compatible with the C+17 standard. This test build shall not be blocking on the CI, rather inform selected persons that Qt does not build anymore with forced C+17 standard. Offending code statements should be reviewed individually