chore(deps): update prom/alertmanager docker tag to v0.26.0

Author: renovate[bot]

files/apps/monitoring/docker-compose.yml

@@ -39,7 +39,7 @@ services:
       - "org.label-schema.group=monitoring"
 
   alertmanager:
-    image: "prom/alertmanager:v0.25.0"
+    image: "prom/alertmanager:v0.26.0"
     container_name: "alertmanager"
     volumes:
       - "./alertmanager:/etc/alertmanager"