Add deprecation notice for Elasticsearch 7 (#2129)

Based on a similar notice we added for v6:
https://docs.hazelcast.com/hazelcast/5.3/integrate/elasticsearch-connector.

Not referencing the specific CVEs announced in the security advisory
because there may be more in future and we don't want to imply an
exhaustive list.

Author: Rob-Hazelcast

docs/modules/integrate/pages/elasticsearch-connector.adoc

@@ -3,6 +3,8 @@
 Elasticsearch is a popular fulltext search engine. You can
 use Elasticsearch as both a source and a sink with the Jet API.
 
+CAUTION: Elastic discontinued support for Elasticsearch 7 in January 2026. This connector is deprecated and will be removed in a future version of Hazelcast.
+
 == Installing the Connector
 
 This connector is included in the full distribution of Hazelcast.