Update ES indexer docs for Omnibus users

What does this MR do?

Updates ES indexer docs 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

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63207

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