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.

[laneholloway]

Fixed in #201

[AMoo-Miki]

The correct header for this file is:

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

Files like this one that have no other attributions should also use the short version above.

For files that contain attribution to an external project, place this long version above those of the external project(s).

/*
 * SPDX-License-Identifier: Apache-2.0
 *
 * The OpenSearch Contributors require contributions made to
 * this file be licensed under the Apache-2.0 license or a
 * compatible open source license.
 *
 * Modifications Copyright OpenSearch Contributors. See
 * GitHub history for details.
 */

PS, updated the long version.

[dlvenable]

Data Prepper Core and Data Prepper API updated in #569