Drop support for Debian 13 'Trixie'

This issue tracks dropping support for building packages for Debian 12, for which upstream support is expected to end on June 2030 ("Debian LTS" on [this page](https://endoflife.date/debian)). However, we're forced to drop support earlier, when support for Python 3.13 - which [trixie is stuck with](https://packages.debian.org/trixie/python/python3) - [ends October 31st, 2029](https://endoflife.date/python)).

As of writing this issue, to remove support for an old Debian distribution:

* remove the distribution from the `DISTS` list. Here's [an example](https://github.com/element-hq/synapse/commit/2c8bf9a3ab0132b430c7e0f859257b79b8b6cb40).

**Do not** remove old distributions from the [pkgs-repo-configs](https://github.com/matrix-org/pkgs-repo-configs) scripts. Otherwise this will break the hosting of old packages.

Maintainers, please check [the latest steps in Element's handbook](https://handbook.element.io/books/backend-team/page/synapse-debian-packages#bkmrk-removing-an-end-of-l) to ensure requirements haven't changed since this issue was written.

Start with scripts-dev/build_debian_packages.py and inspect the DISTS list, then check Element's handbook for the current removal requirements. Resolve the Debian 12 versus Debian 13 discrepancy in the issue before editing. Done means the intended old distribution is removed from DISTS while pkgs-repo-configs remains unchanged.

Written by the indexing model from the issue text.