Add Elasticsearch deprecation note (#23901)

Author: boosty

UPGRADING.md

@@ -146,3 +146,14 @@ The following REST API changes have been made.
 | All `/api/plugins/org.graylog.plugins.datalake/data_lake/...`               | Renamed from `/api/plugins/org.graylog.plugins.datawarehouse/data_warehouse/...`. The corresponding permissions are also renamed to `data_lake...` |
 | All `/api/plugins/org.graylog.plugins.securityapp.asset/assets/history/...` | Removed all endpoints. Contents of underlying `asset_history` MongoDB collection migrated to `Asset History` Index set and Stream               |
 | `GET /<endpoint>`                                                           | description                                                                                                                                        |
+
+## Deprecation of Elasticsearch
+
+Graylog introduced support for OpenSearch as its new search backend in 2022. To simplify the installation and management
+of OpenSearch, the Graylog Data Node component was later developed. Today, Data Node or a self-managed OpenSearch
+deployment are the preferred search backend options for running Graylog.
+
+Starting with Graylog 7.0, the use of Elasticsearch as a search backend is deprecated.
+
+Graylog users are encouraged to migrate to Data Node or self-managed OpenSearch, as Elasticsearch support will be 
+removed entirely in Graylog 8.0.