Update doc/integration/elasticsearch.md

What does this MR do?

The current link to Elasticsearch install points to the current version (v7.3) of Elasticsearch but the table above says we only support v6.x. I am updating the link to point to the last version of 6 instead of 7.3 so users don't have to figure that out on their own.

Once we support 7.x, we can revert this change and simply update the table above to show support of 7.x.

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