Change the build system to enable building with Qt 6

This was built with: -DQT_MAJOR_VERSION=6
-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=5.90.0
-DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x055a00

Split operator<<(QDebug, Attica::Provider const&), in a private header and include where needed; in Qt6 QMetaTypes that have such an operator now have a requirement to make it visible to the linker, otherwise the build fails: /usr/lib64/gcc/x86_64-suse-linux/11/../../../../x86_64-suse-linux/bin/ld: CMakeFiles/providertest.dir/providertest.cpp.o: in function QtPrivate::QDebugStreamOperatorForType<Attica::Provider, true>::debugStream(QtPrivate::QMetaTypeInterface const*, QDebug&, void const*)': /usr/include/qt6/QtCore/qmetatype.h:2216: undefined reference to operator<<(QDebug, Attica::Provider const&)'

(There was a similar issue in KAuth).