Document that we only support Elastic Search 8 (latest major ES version)

[acesdm]

Is your feature request related to a problem? Please describe.
ES 7 as a LEC output fails.

Describe the solution you'd like
Just document that we support only ES 8.

Additional context

2022-06-27 21:18:58 +0000 [error]: #0 config error file="fluentd/etc/fluent.conf" error_class=Fluent::ConfigError error="Using Elasticsearch client 8.2.2 is not compatible for your Elasticsearch server. Please check your using elasticsearch gem version and Elasticsearch server."
The client is unable to verify that the server is Elasticsearch. Some functionality may not be compatible if the server is running an unsupported product.
2022-06-27 21:18:58 +0000 [error]: Worker 0 finished unexpectedly with status 2
2022-06-27 21:18:58 +0000 [info]: Received graceful stop

[camposer]

• Fix version in Gemfile to ES8
• Change LOG_EXPORT_CONTAINER_OUTPUT to use elasticsearch-8 instead of just elasticsearch
• Update docs