This commit modifies .images-debian.yml and README.md to drop
unsupported *-backports releases, starting with stretch, which is no
longer supported. Additionally, the bullseye-backports queue will be
closed soon, as old-stable-backports are only available for 1 year after
a new Debian stable release.

For reference, see this thread:
https://lists.debian.org/debian-backports/2024/07/msg00003.html



Signed-off-by: Aquila Macedo <aquilamacedo@riseup.net>