Move to SQLite imported target and CMake 3.18 for Ubuntu CI

This MR migrate the code to work with the new SQLite imported target.

This is handled by a FindSQLite3.cmake introduced with CMake 3.14, so the MR also updates the Ubuntu Docker image used in the CI to use a more recent version of CMake. The version shipped with Ubuntu with apt is old (3.10), so I install a more recent version by hand in the Ubuntu CI docker image.

This MR is needed for the integration of the new GeoModel monorepo into AtlasExternals.