justfile: Remove unused commands

justfile: Rework upload, multi-arch manifest & sign

justfile: Use arrays for command arguments

justfile: Use 96 max layers for container images

justfile: Compress using zstd for F43+ container images

justfile: Add retry logic to all skopeo/buildah commands

Retry those commands 5 times with a 1 min interval between each try.
Those are the most frequent source of pipeline failures due to quay.io
infra flakes.

generate-gitlab-ci: Add env vars & artifacts handling

generate-gitlab-ci: Drop xfce-atomic & lxqt-atomic

gitlab-ci.yml: Remove generated config

We now automatically generate the GitLab CI config with a script. Thus
remove it from the main repo and keep it only in the branches on the
GitLab repo to reduce rebases and conflicts.