Update release notes link for Elasticsearch client

[wilkinsona]

They've moved, certainly for the 9.0.x client anyway. They're now at https://www.elastic.co/docs/release-notes/elasticsearch/clients/java/9-0-0.

[snicoll]

Unfortunately the publication of releases notes is inconsistent. 9.0.1, 9.0.2, and 9.0.3 don't have one.

(cc @swallez)