logback: suppress zookeeper warnings

Motivation:
ZK logs transient errors at warning log level, thus confusing dCache admins.

```
22 Aug 2024 10:43:19 (System) [] Session 0x10041c40b759013 for server xxxxxxx, Closing socket connection....
org.apache.zookeeper.ClientCnxn$SessionTimeoutException: Client session timed out, have not heard from ....
	at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1251)
```

Modification:
Update logback config to suppress warning messages.

Result:
Less noise in the log files.

Fixes: #7564
Acked-by: Lea Morschel
Target: master
Require-book: no
Require-notes: yes

skel/etc/logback.xml

@@ -300,6 +300,12 @@
         <logger>org.apache.zookeeper.server.NIOServerCnxnFactory</logger>
         <level>error</level>
     </threshold>
+    <threshold>
+      <!-- ZK logs transient errors at warning log level, thus confusing dCache admins.
+          See: https://github.com/dCache/dcache/issues/7564 -->
+      <logger>org.apache.zookeeper.ClientCnxn</logger>
+      <level>error</level>
+    </threshold>
     <threshold>
       <logger>liquibase</logger>
       <level>warn</level>