Elasticsearch: Remove support for EOL 6.x versions

Remove support for Elasticsearch versions 6.[2-4], as they became EOL
recently [1]. Versions 6.{0,1} were never added in for Gerrit support.

[1] https://www.elastic.co/support/eol

Feature: Issue 12527
Change-Id: Ib0971eb98df4586920d364170d17591dd19aeed3
3 files changed
tree: 45d47b23ccd3c698c05cce201f5b855aa39db4a0
  1. java/
  2. javatests/