Upgrade Prometheus and alertmanager

Upgrade Prometheus to 2.4.3 and Alertmanager to 0.15.2

changelogs/unreleased/pl-uprade-prometheus-alertmanager.yml

+---
+title: Upgrade Prometheus to 2.4.3 and Alertmanager to 0.15.2
+merge_request: 22600
+author:
+type: other

vendor/prometheus/values.yaml

 alertmanager:
   enabled: false
+  image:
+    tag: v0.15.2
 
 kubeStateMetrics:
   enabled: true
@@ -16,7 +18,7 @@ rbac:
 server:
   fullnameOverride: "prometheus-prometheus-server"
   image:
-    tag: v2.1.0
+    tag: v2.4.3
 
 serverFiles:
   alerts: {}