Remove wrong include and modernize example

[stephan-cr]

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[czoido]

Hi @stephan-cr, Thanks a lot for the contribution. The duplicated include is ok, it is supposed to include the downloaded conan.cmake in case someone uncomments that block of code. If we remove it it will take the file in the root folder instead of the downloaded one. Also, the cmake_minimum_required should be raised to 3.8 when cxx_std_11 was introduced.

[stephan-cr]

@czoido Resolved the merge conflict.

I would also propose to include conan.cmake like include(conan), because I think that's the point of having list(APPEND CMAKE_MODULE_PATH ${CMAKE_BINARY_DIR}) at the top.