Upgrade to Hibernate Search 6.0.0.CR1 / Elasticsearch 7.9.2

[yrodiere]

Nothing much to report, it's mostly just a version bump.

To be added to the migration guide:

## Hibernate Search ORM + Elasticsearch (Preview)

* Many deprecated methods and classes were removed. For more information: https://in.relation.to/2020/11/04/hibernate-search-6-0-0-CR1/#breaking_changes
* Async/reactive methods now return `CompletionStage` instead of `CompletableFuture`. To convert a `CompletionStage` to a `Future`, call `.toCompletableFuture()`.

[gsmet]

Thanks! Migration guide updated.