Remove cmake_minimum_required call from FindLibzip

[traversaro]

Find<package>.cmake scripts do not contain typically the cmake_minum_required_version call, see for example:

• https://github.com/Kitware/CMake/blob/master/Modules/FindPNG.cmake
• https://github.com/KDE/extra-cmake-modules/blob/master/find-modules/FindOpenEXR.cmake

Furthermore, CMAKE_MINIMUM_REQUIRED_VERSION was removed in https://github.com/SFI-Mechatronics/FMI4cpp/commit/784db15863e7a07b3681dca3289caf638aa92a11

[markaren]

Thanks, must have forgotten to reload the cmake cache and missed this one. Will merge later today. Thanks

[markaren]

Merged, but in the future it's best to target the dev branch