Update Elasticsearch version support for LTS 2021

The compatibility matrix (https://doc.nuxeo.com/nxdoc/compatibility-matrix/) says:

cluster: 7.7 to 7.9 RestClient protocol only. 

but  Elastic ensures forward compatibility on minor version, so any 7.x with x >=9 should work.

Also, 7.7 has been validated to work with Nuxeo.

See https://www.elastic.co/guide/en/elasticsearch/client/java-rest/current/java-rest-high-compatibility.html

Add a note that no test has been done so far with 7.11 or 7.12