ES8 compatibility mode with ES7

[duksis]

Hello there!

Thanks for the project! 🙇

I'm looking into upgrading my server to ES8 and in the documentation
found the following.

If you have an existing application enable the compatibility mode by setting ELASTIC_CLIENT_APIVERSIONING=1 
environment variable. This will instruct the Elasticsearch server to accept and respond with 7.x-compatibile 
requests and responses.

Though having issues on how to configure this within elasticsearch-elixir as it would make the migration process easier.

✌️