retire bookworm images at EOL date

When Debian 12 "bookworm" goes EOL, retire its images from this base repo. This is similar to #19 (closed), but it should be implemented later.

This is technically part of the %Debian 13 trixie upgrade milestone, but we're not assigning it because we want to close the milestone, since the upgrade is essentially done.

We could consider this part of the "forky" release however: bookworm will, after all, go EOL around the time we'll start upgrading to forky, which is currently estimated to be in 2028, according to TPA-RFC-87: container image lifecycle.