remove support for elasticsearch6, add opensearch instead

[wgresshoff]

:heart: Thank you for your contribution!

Description

This pull request removes elasticsearch 6 support and adds support for Opensearch 1.

Checklist

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[ntarocco]

@ppanero do you mind having a quick look?

Thanks @wgresshoff!

[ntarocco]

We will have to Make sure that you have updated the copyright headers for touched files before merging.