Update docs to note ES go indexer included in Omnibus

Currently the official ElasticSearch Integration documentation states that:

Elasticsearch is not included in the Omnibus packages. You will have to install it yourself whether you are using the Omnibus package or installed GitLab from source.

This is no longer the case since 11.8, as confirmed in the ElasticSearch Deep Dive.

Docs should be updated to note:

• Omnibus includes the new Go indexer and all dependencies since 11.8
• building and installing the Go indexer is only required for source (and older) installations