Drop support for Ubuntu 26.04 LTS 'Resolute Raccoon'

This issue tracks dropping support for building packages for Ubuntu 26.04, for which upstream support is expected to end on May 2031 ("End of Standard Support" on [this page](https://ubuntu.com/project/docs/release-team/list-of-releases/)).

As of writing this issue, to remove support for an old Ubuntu 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` 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 Ubuntu 26.04 is removed from the build distribution list without changing the pkgs-repo-configs scripts, while preserving support for hosting old packages.

Written by the indexing model from the issue text.