Thanks for your reply, @ritchierich. We're not using Kibana or any other component of the Elastic Stack.
The warning message is logged by the Java High Level REST client because the Elasticsearch node sends it in its response (Warning HTTP response header) and I want to know how to disable the generation of this warning on the Elasticsearch node. 
I'd also be fine with disabling the log message for this particular deprecation warning in the client, but I didn't find any configuration settings to do so.