Validate Elasticsearch version for compatibility

[ruflin]

Fleet Server is compatible with an Elasticsearch version >= the fleet-server version of the same major. The bugfix version differ. On majors, it is expected that the last minor of fleet-server is compatible with the first minor of the next major Elasticsearch version.

This issue is to document and verify that these restrictions are in place.

Kibana is expected to be on the same version as Elasticsearch. Upgrades work in the following order: Elasticsearch, Kibana, Fleet-server, Elastic Agents.

Few examples (made up versions):

• fleet-server 7.13.0, Elasticsearch 7.13.0: yes
• fleet-server 7.13.2, Elasticsearch 7.13.1: yes
• fleet-server 7.13.2, Elasticsearch 7.14.2: yes
• fleet-server 7.last, Elasticsearch 8.0.0: yes
• fleet-server 7.14.0, Elasticsearch 7.13.0: no
• fleet-server 8.0.0, Elasticsearch 7.last: no