avro

Bump protobuf-java from 3.15.3 to 3.15.6 in /lang/java

Bumps [protobuf-java](https://github.com/protocolbuffers/protobuf) from 3.15.3 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](https://github.com/protocolbuffers/protobuf/compare/v3.15.3...v3.15.6)

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