chore(helm): update image docker.io/bitnami/alertmanager digest to e8d2dc5 (#31614)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
|
[docker.io/bitnami/alertmanager](https://redirect.github.com/bitnami/containers)
([source](https://redirect.github.com/bitnami/containers/tree/HEAD/bitnami/alertmanager))
| digest | `48b6281` -> `e8d2dc5` |

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charts/premium/prometheus/Chart.yaml

@@ -42,4 +42,4 @@ sources:
   - https://hub.docker.com/r/bitnami/prometheus
   - https://quay.io/thanos/thanos
 type: application
-version: 21.2.1
+version: 21.2.2

charts/premium/prometheus/values.yaml

@@ -6,7 +6,7 @@ thanosImage:
   tag: v0.37.2@sha256:4ec6df40fdb921de162677b321057caa8fb8324c65f4cfae9568ed55c4469e2c
 alertmanagerImage:
   repository: docker.io/bitnami/alertmanager
-  tag: 0.28.0@sha256:48b628143248ecb846468dc74761ba814c49ac90ae3d8925b27d1687b5090a1f
+  tag: 0.28.0@sha256:e8d2dc5d84cb56034797b26124703d17b0e026ee72cb385b26aaa4a1d5ade688
 global:
   labels: {}
 workload: