Add Debian 14 'Forky' package support

This issue tracks adding support for building packages for Debian 14, which is expected to be released around Jun-Aug 2027.

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.

Review the latest Debian-package instructions in Element's handbook first. Update debian/conf/distributions and debian/conf/incoming in pkg-repo-configs, then update scripts-dev/build_debian_packages.py in Synapse without removing older distributions. Done means Debian 14 (Forky) packages can be built and hosted alongside existing distributions.

Written by the indexing model from the issue text.