Add Ubuntu 27.04 'NAME TODO' package support

This issue tracks adding support for building packages for Ubuntu 27.04, which is expected to be released around April 2027.

As of writing this issue, to add support for a new Ubuntu distribution, the steps are to update the following files across two repos:

* and
* Note that old distributions **should not** be removed from these lists, 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-adding-a-new-distrib) to ensure requirements haven't changed since this issue was written.

Start by checking the latest Element handbook instructions for adding a Debian distribution. Then update debian/conf/distributions and debian/conf/incoming in pkg-repo-configs, plus scripts-dev/build_debian_packages.py in Synapse, while preserving existing distributions. Done means Ubuntu 27.04 package-building support is configured across both repositories.

Written by the indexing model from the issue text.