Pin pdbmux to v0.4.0

v0.3.0 still serves facts from cache and collapses non-4xx upstream
rejections into a 502.

- Bump the pdbmux image tag to v0.4.0

apps/base/pdbmux/deployment.yaml

@@ -25,7 +25,7 @@ spec:
         - name: pdbmux
           # Image is published by the pdbmux repo's .woodpecker/docker.yaml on
           # a v* tag. It only exists after that tag is cut (see PR merge gates).
-          image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/pdbmux:v0.3.0
+          image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/pdbmux:v0.4.0
           imagePullPolicy: IfNotPresent
           ports:
             - containerPort: 8080