Merge branch 'stable-3.1'

* stable-3.1:
  Update git submodules
  Update git submodules
  Update git submodules
  Update git submodules
  Remove support for discontinued Elasticsearch version 5.6

Change-Id: I8a7812a9f5de6ea4a9c7ff06b92df17065da15fd
tree: a2cd3a28fd5537409e7d118e19f258726c1caf57
  1. java/
  2. javatests/