v-m/alertmanager: Migrate PVC to Synology

We also don't need a 4 GB volume for AlertManager; even 500 MB is
way too big for the tiny amount of data it stores, but that's about the
smallest size a filesystem can be.

victoria-metrics/alertmanager.yaml

@@ -83,4 +83,4 @@ spec:
       - ReadWriteOnce
       resources:
         requests:
-          storage: 4G
+          storage: 500M