Update slapd image to Debian Bullseye.

There's an update to OpenLDAP 2.5 or maybe 2.6 coming down the pipe. But
I'm kicking the can down the road for now. Keeping up with Debian
releases is enough for today.

slapd/Dockerfile

-FROM debian:buster-slim
+FROM debian:bullseye-slim
 # hadolint ignore=DL3008
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \