Add Elasticsearch support information for 6.8 and 7.3
Add Elasticsearch support information for 6.8 and 7.3

Change-Id: I57e756e17411ac1620587e34919364d91fadc36f

GerritCodeReview/homepage,GerritCodeReview/homepage,GerritCodeReview/homepage,GerritCodeReview/homepage,GerritCodeReview/homepage