Hibernate Search is compatible with Elasticsearch server 6.8.
The Elasticsearch client dependency is fixed and you should not deviate from the version Hibernate Search depends on. The Elasticsearch client in version 7.10 will still work with an Elasticsearch server in version 6.8.
The Elasticsearch server (cluster), on the other hand can be in any supported version.
EDIT: Note that I’m referring to the Elasticsearch client that Hibernate Search uses, i.e. the low-level REST client. If you have an additional dependency to the high-level REST client, things get much more complicated, since that client (which Hibernate Search doesn’t use) is very, very coupled to the Elasticsearch server version, so the high-level REST client version 7.10 is unlikely to work with an Elasticsearch server in version 6.8.