Support older CMake versions (#7003)

Support older CMake versions (#7003)

Fixes #7001. Commit c8bc488 introduced the use
of list(PREPEND ...) which was only introduced in version 3.15. We require
CMake 3.9 or later and this commit makes our build system compatible with older
CMake versions again. It also changes our CI to have two builds with Ubuntu
18.04 and two builds with Ubuntu 20.04 to better cover different versions of
build tools (including CMake 3.10).