Elasticsearch: Discontinue EOL versions 7.0 and 7.1 support

Remove support for elasticsearch versions 7.0 and 7.1, as both became
EOL recently [1].

- 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: I6eb757f538b0b529b23324a55c994c3d573163cf
4 files changed