rules: Switch to direct external gitlab-ci for packaging

Rather than having debian/apertis/gitlab-ci.yml on each packaging repository to include the packaging pipeline in infrastructure/ci-package-builder> let's use the remote reference support in the setting.

Once applied, we will then be able to:

1. remove debian/apertis/gitlab-ci.yml from all the packaging branches
2. drop the debian/buster-gitlab-update-job branches and run the pull pipelines on debian/buster

This also includes b2859a6f to easily see what would be changed.

The idea is to manually apply the changes once the review said they are ok, so that's why the pipeline is not succeeding.