Update Elasticsearch training task with latest blog post link

AFAIK, we have three blog posts about the Elasticsearch integration. I am updating the link to the latest one which should have the information about latest development update. Also, at the top of this blog post, it has links to previous two so that people can find the content about where we are from if they are interested. An alternative is to list all three here, but I feel the most recent one is more relevant and also listing all of them could appear overwhelming to readers.