Update license header

[ps48]

Issue:

The current license header needs to be changed from ASF License to:

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

Also, need to update the same in contributing.md document.