AT:

• on startup Optimize retrieves the elasticsearch version via Rest-API and check if this version is actually supported
• Depending on the ES version Optimize has the following behaviour:
  < 6.2: throw error message that ES version is not supported
  6.2 - 6.5: everything is fine. No log output at all or maybe an info message that Optimize is communicating with ES version 6.X
  > 6.5 but < 7.0: log warning that the version might work, but is not officially supported
  >= 7.0: throw error message that ES version is not supported
• if the version is not supported Optimize does not start
• it is documented in the technical guide which versions are supported
• it easy to add/remove supported versions for a new Optimize version