FindProtobuf: C++11 support is required since version 3.6.0

I'm not sure if one should set CXX_STANDARD_REQUIRED also, especially I'm unsure what happens if someone sets e.g. CXX_STANDARD 14 for a executable that this is linked to, and C++14 is not available (i.e. if the 11 is upgraded to 14, but the REQUIRED is kept). Also open is if this also applies to protobuf-lite. And if this should maybe target 3.15.

I would like to see what happens in the dashboards first before doing more on this as it should fix some of my dashboards using older compilers where the default C++ standard is 98.