Update the base image for the haproxy container.

The haproxy version remains the same, but it's no longer necessary
to install from a backports repo.

container/Dockerfile.haproxy

 ARG go_version=1.17
-ARG debian_tag=10.9-slim
+ARG debian_tag=11.1-slim
 ARG dataplane_version=v2.3.1
 
 # Build k8s-crt-dnldr in the builder stage
@@ -22,9 +22,7 @@ RUN cd /go && git clone https://github.com/haproxytech/dataplaneapi.git && \
 # Build haproxy
 FROM debian:${debian_tag}
 
-COPY buster-backports.list /etc/apt/sources.list.d/
-
-ARG haproxy_version=2.2.9-2+deb11u2~bpo10+1
+ARG haproxy_version=2.2.9-2+deb11u2
 
 RUN apt-get update -q && \
     apt-get install -y -q haproxy=${haproxy_version} && \

container/buster-backports.list

-deb http://deb.debian.org/debian buster-backports main