Add Ubuntu 28.04 LTS 'NAME TODO' package support

This issue tracks adding support for building packages for Ubuntu 28.04, which is expected to be released around April 2028.

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.

Read the latest Element handbook steps first, then inspect debian/conf/distributions and debian/conf/incoming in pkg-repo-configs and scripts-dev/build_debian_packages.py in Synapse. Update the relevant distribution entries without removing older ones, and verify that packages can be built and hosted for Ubuntu 28.04.

Written by the indexing model from the issue text.