[data-plane]: Bump protobuf.version from 3.14.0 to 3.15.0 in /data-plane (#659)
[data-plane]: Bump protobuf.version from 3.14.0 to 3.15.0 in /data-plane (#659)

Bumps `protobuf.version` from 3.14.0 to 3.15.0.

Updates `protobuf-java` from 3.14.0 to 3.15.0
- [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/v3.14.0...v3.15.0)

Updates `protobuf-java-util` from 3.14.0 to 3.15.0
- [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/v3.14.0...v3.15.0)

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Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
knative-sandbox/eventing-kafka-broker,knative-sandbox/eventing-kafka-broker,knative-sandbox/eventing-kafka-broker