[FEATURE] Update REST client used in OpenSearchRestTemplate

[curena-contrast]

Is your feature request related to a problem?

The client used in org.opensearch.data.client.orhlc.OpenSearchRestTemplate, org.opensearch.client.RestHighLevelClient, has been deprecated in Elasticsearch since v7.15. The OpenSearch documentation also considers it deprecated.

What solution would you like?

The OpenSearchRestTemplate should use the Java client org.opensearch.client.opensearch.OpenSearchClient instead.

What alternatives have you considered?

Continuing to use the deprecated client runs the risk of breakage through its removal.

Do you have any additional context?

No

[curena-contrast]

Note to maintainers:
Consider adding an issue type that's neither a bug nor an enhancement -- E.g., Technical debt, wherein this particular issue would be more accurately categorized.

[curena-contrast]

And thanks for this fabulous Spring Data implementation!

[dblock]

@curena-contrast Want to take this on?

[reta]

@curena-contrast this is a duplicate of #19