debusine: Enable trixie-backports
We don't need bookworm-backports any more, but freexian-team/debusine!2492 will need python-django-pgtransaction from trixie-backports.
We don't need bookworm-backports any more, but freexian-team/debusine!2492 will need python-django-pgtransaction from trixie-backports.