Add changelog for 8.4.0

Co-authored-by: Seth Michael Larson <seth.larson@elastic.co>

Author: github-actions[bot]

docs/changelog.asciidoc

@@ -1,6 +1,15 @@
 [[changelog-client]]
 == Release notes
 
+[discrete]
+=== 8.4.0
+
+[discrete]
+===== Support for Elasticsearch `v8.4.0`
+
+You can find all the API changes
+https://www.elastic.co/guide/en/elasticsearch/reference/8.4/release-notes-8.4.0.html[here].
+
 [discrete]
 === 8.2.1