Merge branch 'stable-2.16' into stable-3.0

* stable-2.16:
  Remove support for discontinued Elasticsearch version 5.6

Change-Id: I165b4d4fc9d3589ba03121bb9c25b5ff60459124
tree: 024eeb533e0c4665c066559110300412d9aa26a5
  1. java/
  2. javatests/