Changes

testenv: Dockerfile: fix Suites: stable-backports

Using "stable-backports" broke now that Debian 13 is out. Use
"bookworm-backports" instead.

Fix for:
  E: The value 'bookworm-backports' is invalid for APT::Default-Release as such a release is not available in the sources

Change-Id: Icc46b6dc2493806dc12f0ff442abd70fe5b48eb8