branching: add new job ci-package-builder-update requesting to update ci-package-builder for stable release only

Adding the new release in a list DOWNSTREAM_RELEASES_debian_* ensures
the ci-package-builder pipeline will correctly assign merge requests
updating packages. Otherwise, the pipeline may create merge against
wrong branches.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>