Elasticsearch v8 support/ Updated documentation

[andrew-pickin-epi]

• [x] read the contribution guideline

Difficultly in planning a production migration to Elasticseach version 8+

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 https://github.com/uken/fluent-plugin-elasticsearch/issues/937 marked as "Future Work".

Updates to supprt Elasticsearch (if required) and updated documentaion.

[github-actions[bot]]

@andrew-pickin-epi this issue was automatically closed because it did not follow the issue template.