Bump Google.Protobuf in /examples/csharp/Xds/Greeter (#28858)
Bump Google.Protobuf in /examples/csharp/Xds/Greeter (#28858)

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) from 3.12.2 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/v3.12.2...v3.15.0)

---
updated-dependencies:
- dependency-name: Google.Protobuf
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
vjpai/grpc,ejona86/grpc,vjpai/grpc,nicolasnoble/grpc,vjpai/grpc,ctiller/grpc,grpc/grpc,nicolasnoble/grpc,stanley-cheung/grpc,vjpai/grpc,vjpai/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,jtattermusch/grpc,ejona86/grpc,ejona86/grpc,stanley-cheung/grpc,vjpai/grpc,grpc/grpc,grpc/grpc,grpc/grpc,ejona86/grpc,ctiller/grpc,grpc/grpc,nicolasnoble/grpc,ejona86/grpc,vjpai/grpc,jtattermusch/grpc,ctiller/grpc,jtattermusch/grpc,vjpai/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,stanley-cheung/grpc,vjpai/grpc,jtattermusch/grpc,grpc/grpc,ctiller/grpc,ctiller/grpc,nicolasnoble/grpc,stanley-cheung/grpc,ejona86/grpc,nicolasnoble/grpc,ejona86/grpc,jtattermusch/grpc,ctiller/grpc,nicolasnoble/grpc,ejona86/grpc,stanley-cheung/grpc,ctiller/grpc,stanley-cheung/grpc,vjpai/grpc,nicolasnoble/grpc,stanley-cheung/grpc,stanley-cheung/grpc,nicolasnoble/grpc,jtattermusch/grpc,ctiller/grpc,ctiller/grpc,vjpai/grpc,nicolasnoble/grpc,nicolasnoble/grpc,ejona86/grpc,grpc/grpc,stanley-cheung/grpc,ejona86/grpc,vjpai/grpc,jtattermusch/grpc,jtattermusch/grpc,grpc/grpc,nicolasnoble/grpc,ctiller/grpc,ctiller/grpc,ejona86/grpc,ctiller/grpc,ejona86/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,jtattermusch/grpc,nicolasnoble/grpc,grpc/grpc