Validate Elastic Agent compatibility

[ruflin]

Currently Elastic Agent talks to Kibana and verifies the version. In the future this will fleet-server. For Elastic Agent there are 2 compatibility parts:

• Standalone: Compatibility with Elasticsearch
• Managed: Compatibility with fleet-server

As the compatibility with Elasticsearch in the managed mode is enforced by fleet-server (#167) this should not be an issue. In general the compatibility looks as following:

Elastic Agent <= fleet-server <= Elasticsearch

The bugfix release might differ. The last minor or Elastic Agent in a major should be compatible with the first minor of the next major of fleet-server. Same for Elasticsearch. This means fleet-server / Elasticsearch must be upgraded first.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)