Add Ubuntu 28.10 'NAME TODO' package support

This issue tracks adding support for building packages for Ubuntu 28.10, which is expected to be released around October 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.

Check the latest Element handbook instructions for adding a Debian package distribution first. Then inspect pkg-repo-configs/debian/conf/distributions, pkg-repo-configs/debian/conf/incoming, and Synapse's scripts-dev/build_debian_packages.py; update the Ubuntu 28.10 entries without removing older distributions, and verify that package builds target the new distribution.

Written by the indexing model from the issue text.