C++ 17 support for Qt 6

[MartinDelille]

One little question: when building with Qt6, how do you handle c++ version ? You require c++ 11 here: https://github.com/QtExcel/QXlsx/blob/master/QXlsx/CMakeLists.txt#L14 but Qt 6 needs C++ 17.

I had to patch your CMakeLists.txt to make it work on the conan recipe: https://github.com/conan-io/conan-center-index/pull/11364/files

[MartinDelille]

I submitted a PR to fix this problem: #223