Merge branch 'stable-3.1' into stable-3.2

* stable-3.1:
  Update git submodules
  Update git submodules
  Compact the REST-API output JSON unconditionally
  Elasticsearch: Discontinue EOL versions 7.0 and 7.1 support
  Elasticsearch: Discontinue EOL version 6.8 thus whole V6 support
  Update git submodules
  Avoid logging "length=0" exception

Change-Id: If2b3e33e574b8cb8672eed76edb7de31c70a8223
tree: fa6a667e0ee7668105eacc037f305a480264799b
  1. java/
  2. javatests/