fix: Revert edX Forum container to use Bullseye base image

There is no 3.0 release of the Ruby image based on the Bookworm Debian release, and
Forum doesn't work with newer versions of Ruby (yet).

dockerfiles/openedx-forum/Dockerfile

@@ -1,4 +1,4 @@
-FROM docker.io/ruby:3.0-slim-bookworm
+FROM docker.io/ruby:3.0-slim-bullseye
 ENV DEBIAN_FRONTEND=noninteractive
 RUN apt update && \
   apt upgrade -y && \