What is the bug?
Some OpenSearch java files have different licence formats.

What is the expected behaviour?
Files should have

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

for license header

Files impacted

• core/src/main/java/org/opensearch/sql/expression/aggregation/StdDevAggregator.java
• core/src/main/java/org/opensearch/sql/expression/aggregation/VarianceAggregator.java
• core/src/test/java/org/opensearch/sql/expression/aggregation/StdDevAggregatorTest.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/CompositeAggregationParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/FilterParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/MetricParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/MetricParserHelper.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/NoBucketAggregationParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/OpenSearchAggregationResponseParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/SingleValueParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/StatsParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/Utils.java
• DEVELOPER_GUIDE.rst

Each should be investigated and fixed if possible.