Add support for Elasticsearch version 6.8.x

Update the supported versions to include 6.8.x and update the tests
for version 6.x to use this version.

The latest release version is 6.8.2 but the latest available image
provided by blacktop is 6.8.1 so we use that image for now. When the
latest version is available, we can update the test container in a
follow-up commit.

Bug: Issue 11266
Change-Id: I5aab85b345a2eb11c13d8a36c46ca19ef5c7cefb
8 files changed
tree: ce78c47a6319f63554c3b845fd876a7bdd174c0e
  1. gerrit-acceptance-tests/
  2. gerrit-elasticsearch/
  3. gerrit-pgm/