WIP: Update ES docs to note go indexer included in Omnibus

What does this MR do?

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.

This MR updates the documentation to note that:

• 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

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #63207 (closed)