Remove Stretch images from pipeline (and add Bookworm?)
The normal Debian Stretch repositories (http://deb.debian.org/debian stretch/main) seem to have reached their end of life, therefore the pipeline fails (see e.g. Job #257558). Therefore we should remove the Stretch images from the pipeline. Accordingly, it makes sense to add Bookworm (Debian 12) in turn.
@fgoth Are you planning to add Bookworm images to Z03/pdi?