CI: Disable Debian 9 runner
Debian 9 with GCC 6 does not support the new (now allowed) C++17 Features and is already disabled in the dune-ci docker repo, thus the image is not updated any more.
Edited by Patrick Jaap
Debian 9 with GCC 6 does not support the new (now allowed) C++17 Features and is already disabled in the dune-ci docker repo, thus the image is not updated any more.