[ci] Update ci configuration
The image dune:git-ubuntu-20.04-clang-10-20 was dropped
in favour of dune:git-ubuntu-20.04-clang-10-17 with
docker/ci!128 (merged).
The image dune:git-ubuntu-20.04-clang-10-20 was dropped
in favour of dune:git-ubuntu-20.04-clang-10-17 with
docker/ci!128 (merged).