chore(deps): update container image bitnami/alertmanager to 0.27.0@be756c5 by renovate (#21812)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [bitnami/alertmanager](https://togithub.com/bitnami/containers)
([source](https://togithub.com/bitnami/containers/tree/HEAD/bitnami/alertmanager))
| digest | `69daa81` -> `be756c5` |

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This PR has been generated by [Renovate
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<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzNy4zNTMuMCIsInVwZGF0ZWRJblZlciI6IjM3LjM1My4wIiwidGFyZ2V0QnJhbmNoIjoibWFzdGVyIiwibGFiZWxzIjpbImF1dG9tZXJnZSIsInVwZGF0ZS9kb2NrZXIvZ2VuZXJhbC9ub24tbWFqb3IiXX0=-->

charts/premium/prometheus/Chart.yaml

@@ -43,4 +43,4 @@ sources:
   - https://hub.docker.com/r/bitnami/prometheus
   - https://hub.docker.com/r/bitnami/alertmanager
 type: application
-version: 18.2.0
+version: 18.2.1

charts/premium/prometheus/values.yaml

@@ -6,7 +6,7 @@ thanosImage:
   tag: v0.35.0@sha256:fa1d28718df00b68d6ad85d7c7d4703bd9f59e5cd8be8da6540ea398cf701a1f
 alertmanagerImage:
   repository: bitnami/alertmanager
-  tag: 0.27.0@sha256:69daa81230a814569e5180b0d6ea1a77054fa36589a4d0c823153c7074f6c142
+  tag: 0.27.0@sha256:be756c58a16f7298a1dd6d3b8b52b625627c57f846331174893fe859b0e0ba08
 global:
   labels: {}
 workload: