Drop support for Ubuntu 26.10 'Stonking Stingray' packages

This issue tracks dropping support for building packages for Ubuntu 26.10, for which upstream support is expected to end on July 2027 ("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](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 the latest package-removal requirements in Element's handbook. Remove Ubuntu 26.10 from the build distribution list without changing pkgs-repo-configs; done means old package hosting remains supported while new builds no longer target it.

Written by the indexing model from the issue text.