fix Kafka config mount: use subPath to avoid /etc/kafka/secrets conflict

Container fails with 'read-only file system' when K8s tries to mount
a secret into /etc/kafka/secrets alongside the ConfigMap at /etc/kafka.
Switch to subPath mount of server.properties directly.

apps/base/customer1/siriusdevops-db/kafka-broker.yaml

@@ -99,7 +99,8 @@ spec:
                 - ALL
           volumeMounts:
             - name: config
-              mountPath: /etc/kafka
+              mountPath: /etc/kafka/server.properties
+              subPath: server.properties
             - name: data
               mountPath: /var/lib/kafka/data
           readinessProbe: