Download only shallow copy of external git repos

This speeds up the download as well as lowers the needed disk space. Increase minimum required CMake version to remove warnings when using newer CMake versions.

added Build System label

requested review from @p.-a.loizeau

assigned to @f.uhlig

added Externals label

closed

The shallow download only works when the head of a branch is used. If one tries to download a specific commit which is not at the head one can't use the shallow download feature. The feature such can't be used.

Don't merge the MR but close it with some comment