Add Ubuntu 29.04 'NAME TODO' package support

This issue tracks adding support for building packages for Ubuntu 29.04, which is expected to be released around April 2029.

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 instructions for adding a distribution, then inspect debian/conf/distributions and debian/conf/incoming in pkg-repo-configs and scripts-dev/build_debian_packages.py in Synapse. Update the named locations without removing older distributions, and verify that Ubuntu 29.04 package building is supported across both repositories.

Written by the indexing model from the issue text.