[alertmanager] Update dependency prometheus/alertmanager to v0.29.0 (#6293)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

charts/alertmanager/Chart.yaml

@@ -6,9 +6,9 @@ icon: https://raw.githubusercontent.com/prometheus/prometheus.github.io/master/a
 sources:
   - https://github.com/prometheus/alertmanager
 type: application
-version: 1.27.1
+version: 1.28.0
 # renovate: github-releases=prometheus/alertmanager
-appVersion: v0.28.1
+appVersion: v0.29.0
 kubeVersion: ">=1.25.0-0"
 keywords:
   - monitoring