In Optimize, we're only supporting those Elasticsearch versions that are still maintained by Elastic. See https://www.elastic.co/support/eol. With 3.3, the Elasticsearch 7.0, 7.1, and 7.2 reach their end of life. We should remove them from the support.
- the jenkins pipeline for the ES support is adjusted to discontinuing the testing for 7.0. 7.1, and 7.2
- there's an error thrown if Optimize is connected to one of the unsupported Elasticsearch versions
- the documentation is adjusted (supported environments, announcements, upgrade guide)