chore(helm): update image public.ecr.aws/bitnami/alertmanager digest to a28f966 (#31801)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
|
[public.ecr.aws/bitnami/alertmanager](https://redirect.github.com/bitnami/containers)
([source](https://redirect.github.com/bitnami/containers/tree/HEAD/bitnami/alertmanager))
| digest | `e8d2dc5` -> `a28f966` |

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charts/premium/prometheus/Chart.yaml

@@ -35,11 +35,11 @@ maintainers:
     url: https://truecharts.org
 name: prometheus
 sources:
+  - https://gallery.ecr.aws/bitnami/alertmanager
+  - https://gallery.ecr.aws/bitnami/prometheus
   - https://github.com/prometheus-community/helm-charts
   - https://github.com/prometheus-operator/kube-prometheus
   - https://github.com/truecharts/charts/tree/master/charts/premium/prometheus
-  - https://hub.docker.com/r/bitnami/alertmanager
-  - https://hub.docker.com/r/bitnami/prometheus
   - https://quay.io/thanos/thanos
 type: application
-version: 21.3.0
+version: 21.3.1

charts/premium/prometheus/values.yaml

@@ -6,7 +6,7 @@ thanosImage:
   tag: v0.37.2@sha256:4ec6df40fdb921de162677b321057caa8fb8324c65f4cfae9568ed55c4469e2c
 alertmanagerImage:
   repository: public.ecr.aws/bitnami/alertmanager
-  tag: 0.28.0@sha256:e8d2dc5d84cb56034797b26124703d17b0e026ee72cb385b26aaa4a1d5ade688
+  tag: 0.28.0@sha256:a28f966e3eb374778401554699fc4001f0635d30825f24ef2cc772dd2766ea52
 global:
   labels: {}
 workload: