add dependency note in docs

Author: l-trotta

docs/release-notes/9-0-0.md

@@ -329,7 +329,10 @@ ElasticsearchClient esClient = ElasticsearchClient.of(b -> b
         .apiKey(apiKey)
 );
 ```
-The classic version of the builder is still available.
+The classic version of the builder is still available, both with the new `Rest5Client` and the legacy `RestClient`; while the `Rest5Client` is included with the java client's dependency, the legacy `RestClient` must be imported separately:
+```
+implementation("org.elasticsearch.client:elasticsearch-rest-client:9.0.0")
+```
 ::::
 
 ::::{dropdown} New Rest5Client