Bump natsio/prometheus-nats-exporter in /helm/charts/nats

Bumps natsio/prometheus-nats-exporter from 0.12.0 to 0.13.0.

---
updated-dependencies:
- dependency-name: natsio/prometheus-nats-exporter
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

helm/charts/nats/values.yaml

@@ -378,7 +378,7 @@ promExporter:
   enabled: false
   image:
     repository: natsio/prometheus-nats-exporter
-    tag: 0.12.0
+    tag: 0.13.0
     pullPolicy:
     registry: