[BUG] Incorrect license headers

[dblock]

Is your feature request related to a problem?
Coming from opensearch-project/opensearch-plugins#138, some obsolete ALv2 headers are still present, e.g. here.

What solution would you like?

1. Add automated license header Gradle checks to CI similar to this.
2. Update the default header for all new files created by IDEs similar to [License] Update SPDX License Header OpenSearch#510.
3. Fix any incorrect headers.

Do you have any additional context?
Licenses must match https://github.com/opensearch-project/opensearch-plugins/blob/main/HEADERS.md