Set up CI settings for packaging repositories

Introduce the settings key to edit the project properties and use it to configure the CI settings needed by the packaging repositories:

rules:
  - matches:
      - pkg/*
    settings:
      ci_config_path: debian/apertis/gitlab-ci.yml
      lfs_enabled: yes

For the list of supported properties, check https://docs.gitlab.com/ee/api/projects.html#edit-project

The default_branch key is now deprecated: use the new mechanism to set it.