Trivial: standardise on netcat-openbsd.

Shouldn't matter much but might as well keep this consistent (both with
what we used to install under 22.04 and with what's in
govuk-infrastructure/toolbox).

The OpenBSD implementation lacks `-e` (which is a plus) and is a bit
more modern and actively maintained.

netcat is used in the healthcheck script in the underlying clamav
container:
https://github.com/Cisco-Talos/clamav-docker/blob/main/clamav/1.3/debian/scripts/clamdcheck.sh

Dockerfile

@@ -125,7 +125,7 @@ RUN apt update && apt-get install -y libbz2-1.0 \
         libxml2 \
         zlib1g \
         tzdata \
-        netcat-traditional && \
+        netcat-openbsd && \
     mkdir -p /var/run/clamav /var/lib/clamav /usr/local/share/clamav && \
     chown app:app /var/run/clamav /var/lib/clamav /usr/local/share/clamav