Add Ubuntu 27.10 'NAME TODO' package support

This issue tracks adding support for building packages for Ubuntu 27.10, which is expected to be released around October 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.

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. Confirm the current Ubuntu 27.10 codename and update the listed configuration without removing older distributions. Done means the package-building configuration supports Ubuntu 27.10 across both repositories.

Written by the indexing model from the issue text.