After dealing with the aftermath of T362567, we realized that we probably don't have a valid and good reason for including backports to the base images themselves. They rarely, if ever, get used directly and can cause issues like the linked task.

Given that

• backports are pinned to lower priority so packages in them that have a package already in the main component of the version of Debian aren't going to be installed unless explicitly specified
• That blubber has specific support for adding repos like backports

we would like to experiment with dropping backports from the base images and moving usages of it to select repositories blubber.yaml files or any other applicable method.