Breakage with CMake 4

CMake 4 raised minimum compatible version to CMake 3.5.
See https://gitlab.kitware.com/cmake/cmake/-/issues/26698 and https://gitlab.kitware.com/cmake/cmake/-/issues/26613

Currently, there's minimum version of CMake being set at 2.8
https://github.com/opencv/opencv/blob/85669309222191ccf9ed8767ec9f14f0e790eee4/cmake/OpenCVGenPkgconfig.cmake#L113

This breaks compilation.