Correct copyright notices to reflect Copyright OpenSearch Contributors

[dblock]

Coming from opensearch-project/.github#21.

The correct copyright for open-source projects in opensearch-project is "Copyright OpenSearch Contributors". Please correct any places that say otherwise, especially where it says copyright Amazon. Make sure NOTICE.txt and README match. See opensearch-project/.github#24 for an example.

[dblock]

The NOTICE file contents don't match.

Also, isn't this code that came from Elastic? It needs to retain those copyrights in NOTICE.txt, from https://github.com/logstash-plugins/logstash-output-elasticsearch/blob/master/NOTICE.TXT

[AMoo-Miki]

The correct header for this one is:

/*
 * Copyright OpenSearch Contributors
 * SPDX-License-Identifier: Apache-2.0
 */

[VijayanB]

@naveentatikonda can you update this as well? Thanks.