This is a Docker-only update to mitigate [CVE-2023-4863](https://cve.org/CVERecord?id=CVE-2023-4863), a critical vulnerability in `libwebp`. Server admins not using Docker should ensure that their `libwebp` is up to date (if installed). We encourage admins to upgrade as soon as possible. - Update docker image to use Debian bookworm as the base. ([\#16324](https://github.com/matrix-org/synapse/issues/16324))