Add Debian 16 'NAME TODO' package support

This issue tracks adding support for building packages for Debian 16, which is expected to be released around Jun-Aug 2031.

As of writing this issue, to add support for a new Debian 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 for adding a Debian distribution, then inspect debian/conf/distributions and debian/conf/incoming in pkg-repo-configs and scripts-dev/build_debian_packages.py in Synapse. Add Debian 16 support while preserving existing distributions and verify that the package-building configuration recognizes it.

Written by the indexing model from the issue text.