docs: add change log for PR #12492

changes/ce/fix-12492.en.md

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+Return `Receive-Maximum` in `CONNACK` for MQTT v5 clients.
+
+EMQX takes the min value of client's `Receive-Maximum` and server's `max_inflight` config as the max number of inflight (unacknowledged) messages allowed.
+Prior to this fix, the value was not sent back to the client in `CONNACK` message.