Drop support for Ubuntu 27.04 'NAME TODO' packages

This issue tracks dropping support for building packages for Ubuntu 27.04, for which upstream support is expected to end on Jan 2028 (**extrapolating** from "End of Standard Support" fields on [this page](https://ubuntu.com/project/docs/release-team/list-of-releases/) of previous `*.04` 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.

Read the latest Element handbook steps first, then inspect scripts-dev/build_debian_packages.py and its DISTS list. Remove Ubuntu 27.04 from that list when support ends, while leaving the pkgs-repo-configs scripts unchanged so old packages remain hosted.

Written by the indexing model from the issue text.