Enterprise license breaks incompatible clients

[pebrc]

As of version 7.6 of Elasticsearch we prefer an Enterprise stack level license.

But older clients e.g. Beats up to and including version 7.5 do not use the new _license API but call the _xpack API which is not backwards compatible and exposes the concept of Enterprise license to client. The client is not expecting this value and errors out.

A fix for Elasticsearch is underway but even if it lands in 7.8.1 and 7.9.0 we still have versions 7.6.x and 7.7.x that potentially break if users combine them with older versions of Beats.

Two options come to mind:

• document the problem and ask users to upgrade the clients to version > 7.5
• try to fix it by preferring a platinum license which comes with its own set of problems
  1. we could potentially detect if any ECK managed beat is running an incompatible version and provision a platinum license instead of an Enterprise license but we cannot easily detect the version of clients that are not managed by ECK
     • impossible to get correct given that clients outside of ECK's control exist
  2. we could just generally prefer a platinum license for any Elasticsearch version that is less than 7.8.1 or 7.9.0 (this would only be a an issue if we wanted to support Endpoint security for those versions)
     • simplest solution, my favourite
  3. we could make it configurable per cluster for users that use older clients e.g. via label but default to Enterprise
     • comparatively complex implementation and would require us to maintain this for many versions of ECK to come, needs documentation and customers could still run into the issue if they don't read the docs.