Qt6: resolve TODOs against missing KF6

- On FreeBSD, no KF6 was available
- On KDE Neon Unstable, there are somewhat wonky KF6 packages available
- Adjust CMake to find the KDE Neon versions, then fix the C++ code

src/libcalamares/utils/StringExpander.cpp

@@ -11,16 +11,6 @@
 #include "StringExpander.h"
 #include "Logger.h"
 
-#if QT_VERSION >= QT_VERSION_CHECK( 6, 0, 0 )
-// TODO: Qt6
-KWordMacroExpander::~KWordMacroExpander() {};
-bool
-KWordMacroExpander::expandMacro( const QString& str, QStringList& ret )
-{
-    return false;
-}
-#endif
-
 namespace Calamares
 {
 namespace String