Drop support for Debian 12 'Bookworm'

This issue tracks dropping support for building packages for Debian 12, for which upstream support is expected to end on June 2028 ("Debian LTS" on [this page](https://endoflife.date/debian)). However, we're forced to drop support earlier, when support for Python 3.11 - which [bullseye is stuck with](https://packages.debian.org/bookworm/python/python3) - [ends October 31st, 2027](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 review the linked example commit and the latest Element handbook steps. Done means Debian 12 is no longer targeted by the package build while old-package hosting in pkgs-repo-configs remains unchanged.

Written by the indexing model from the issue text.