Fix "Compatibility with CMake < 3.10 will be removed" warnings in gitsafeclone.txt.in

[traversaro]

CMake started complaining for projects that requested 3.5 as cmake_minimum_required (see https://gitlab.kitware.com/cmake/cmake/-/commit/55778f5a16ffe1b67123a31f6b568f2f7c0a1f36), so it make sense to bump the cmake_minimum_required used in gitsafeclone to 3.16 (as the general one of YCM).

This will fix all the warnings:

CMake Deprecation Warning at /home/runner/work/robotology-superbuild/robotology-superbuild/b/src/OsqpEigen/CMakeFiles/YCMTmp/OsqpEigen-gitsafeclone.cmake:8 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

that occur when using a YCM-based superbuild with a recent CMake version.