Set YCM_USE_CMAKE_NEXT to OFF by default to use upstream CMake's ExternalProject instead of YCM's fork

[traversaro]

Historically, ycm-cmake-modules have been carrying on a patched version of CMake's ExternalProject module that made sure that it was never possible for ExternalProject to delete a source directory of a repo (the one contained in robotology-superbuild/src/<package>) if one had local modifications and it did not set YCM_EP_DEVEL_MODE_<package> to ON . For more details, see:

• The part of CMake code that actually deletes a folder: https://gitlab.kitware.com/cmake/cmake/-/blob/v3.30.0/Modules/ExternalProject/shared_internal_commands.cmake?ref_type=heads#L1005-L1029).
• The long discussion of this YCM local modification: Clean ExternalProject patch mess ycm-cmake-modules#50

Unfortunately, the modification was never merged upstream, and it is not anymore possible for me to keep the forked ExternalProject version aligned with the CMake's upstream ExternalProject. So in this PR we set YCM_USE_CMAKE_NEXT to OFF to stop using YCM's ExternalProject module, and start using the regular CMake's ExternalProject.

Fix #1671 .

Important

Before this PR the robotology-superbuild raised an error if you called make update-all or ninja update-all and there was repo with local modification with YCM_EP_DEVEL_MODE_<package> set to OFF. After this PR, instead the robotology-superbuild will discard the local modifications. To avoid losing data, never call the update-all target if you have local modifications in a package and you did not set YCM_EP_DEVEL_MODE_<package> to ON for that package.

If you experience problem with the default setting of YCM_USE_CMAKE_NEXT set to OFF, you can manually set it to ON to restore the behavior of the robotology-superbuild before August 2024. However, this option will be removed in January 2025.