Material: fix build failure with Qt6

src/Mod/Material/App/MaterialLibrary.cpp

@@ -81,7 +81,9 @@ Material* MaterialLibrary::saveMaterial(Material& material, const QString& path,
 
     if (file.open(QIODevice::WriteOnly | QIODevice::Text)) {
         QTextStream stream(&file);
+#if QT_VERSION < QT_VERSION_CHECK(6,0,0)
         stream.setCodec("UTF-8");
+#endif
         stream.setGenerateByteOrderMark(true);
 
         // Write the contents