Elasticsearch: Discontinue EOL version 6.8 thus whole V6

- Remove the whole V6 support as 6.8 was the last supported 6.x version.
- Update the elasticsearch home page accordingly.
- Add that removal information to each next 3.x.y release notes.

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

Change-Id: Ia535d699e87fba3561df9c3182fbda910f04eccb
4 files changed