Bump protobuf from 3.14.0 to 3.15.0

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) 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](protocolbuffers/protobuf@v3.14.0...v3.15.0)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]