artifactapi: use canonical upstream image name for redis_exporter (#429)

rke2's `registries.yaml` already rewrites upstream image names to the artifactapi mirror, so manifests must carry canonical upstream names. Only in-house `artifactapi.k8s.syd1.au.unkin.net/docker-internal/...` images stay explicit.

Changes:
- Point the artifactapi redis metrics sidecar at `docker.io/oliver006/redis_exporter:v1.89.0`.

Tag unchanged. `kustomize build --enable-helm apps/overlays/au-syd1/artifactapi` differs from main only in that image string. No extra proxied refs in the file (the `redis:7-alpine` container is already a bare upstream name).

Reviewed-on: #429
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/artifactapi/redis-deployment.yaml

@@ -54,7 +54,7 @@ spec:
             successThreshold: 1
             timeoutSeconds: 5
         - name: metrics-exporter
-          image: artifactapi.k8s.syd1.au.unkin.net/dockerhub/oliver006/redis_exporter:v1.89.0
+          image: docker.io/oliver006/redis_exporter:v1.89.0
           imagePullPolicy: IfNotPresent
           ports:
             - containerPort: 9121