Elastic search Version 8+ support/Updated documentation

[andrew-pickin-epi]

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Problem

Elasticsearch version 8 comes with breaking changes post version 7.17.
The documnetation does not make it clear whether Elasticsaech version 8+ is supported or what specific configuration options are required.
The situation is made more confusing as support for the deprecation of elasticsearch-xpack #937 marked as "Future Work"

Steps to replicate

N/A

Expected Behavior or What you need to ask

Full support for connection to ES v8+ with suitable documentation.

Using Fluentd and ES plugin versions

Fluentd v1.15
ES: v8+