gitea: use canonical upstream image name for redis_exporter (#431)

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 gitea valkey metrics sidecar at `docker.io/oliver006/redis_exporter:v1.89.0`.

Tag unchanged. `kustomize build --enable-helm apps/overlays/au-syd1/gitea` differs from main only in that image string. No extra proxied refs in the file (the `valkey/valkey:8-alpine` container is already a bare upstream name).

Reviewed-on: #431
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/gitea/valkey-deployment.yaml

@@ -83,7 +83,7 @@ spec:
             - mountPath: /data
               name: data
         - 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