docs: add changelog for PR 13896

changes/ee/fix-13896.en.md

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+Upgrade pulsar client from `0.8.3` to `0.8.4` (see [pulsar#61](https://github.com/emqx/pulsar-client-erl/pull/61).
+
+Piror this fix, if the producer client experiences a 'socket error' (but not a normal 'socket close'), it may continue sending data to a clsoed socket without any error handing.
+From EMQX dashboard, user observe that the 'totoal' counter keep increasing, but not the 'success' counter.