Update Elasticsearch repository indexer related documentation

Problem to solve

The Elasticsearch repository indexer description is out-of-date. We need to update it based on the current state.

Further details

The new elasticsearch indexer written in Golang has been shipped as default and it's not in beta any more. So, we need to update the documentation accordingly.

Proposal

• Update the Elasticsearch indexer part based on its current state
• Check the Elastisearch integration document to make sure other parts are also update-to-date.

Who can address the issue

@toupeira may know the correct state of the indexer.

Other links/references

• Remove Ruby elasticsearch repository indexer
• Elasticsearch integration
• gitlab-elasticsearch-indexer
• Install GitLab-Elasticsearch-indexer