Fix Response\Elasticsearch::offsetGet return type deprecation

[marcortola]

"ArrayAccess::offsetGet()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Elastic\Elasticsearch\Response\Elasticsearch" now to avoid errors or add an explicit @return annotation to suppress this message.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service[bot]]

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[AnnaNtagiou]

@marcortola could you please sign the CLA and rebase?