[CMake] OGS_USE_NETCDF defaults to OFF.

We use NetCDF-C++ which since 8.1.2 is no longer part of VTK:
https://gitlab.kitware.com/vtk/vtk/commit/e3cfcfbc1a65ca0e6896641973b6514f7e726a06

We should refactor our netcdf api usage and maybe provide Conan packages
if required.