Merge branch 'stable-3.1' into stable-3.2

* stable-3.1:
  Fix documentation of `container.slave`
  Update git submodules
  Add support for Elasticsearch version 7.8.*

Change-Id: I2492d62589c7dc959dd5d0c900e3b4abdf92d3ef
tree: 66aa7d5799fc1d565e40a746cbadcb864250f42b
  1. java/
  2. javatests/