Get rid of startup warning message about UNRELEASED ECS Schema v8

[herrBez]

Each time a Logstash instance in version 8 configured with the Elasticsearch Plugin starts, it logs this Warning message:

[2023-03-21T15:56:51,383][WARN ][logstash.outputs.elasticsearch][another_test] Elasticsearch Output configured with `ecs_compatibility => v8`, which resolved to an UNRELEASED preview of version 8.0.0 of the Elastic Common Schema. Once ECS v8 and an updated release of this plugin are publicly available, you will need to update this plugin to resolve this warning.

This is caused by this if block (reported here for convenience):

if ecs_compatibility == :v8
      @logger.warn("Elasticsearch Output configured with `ecs_compatibility => v8`, which resolved to an UNRELEASED preview of version 8.0.0 of the Elastic Common Schema. " +
                   "Once ECS v8 and an updated release of this plugin are publicly available, you will need to update this plugin to resolve this warning.")
end

Given that ECS 8 is a long standing reality, we can (IMO) safely deactivate this warning message (i.e., remove the if-block) to reduce the noise in Logstash Logs.

[mashhurs]

It was addressed with this change: #1131