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
...

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examples/csharp/Xds/Greeter/Greeter.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.12.2" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.0" />
     <PackageReference Include="Grpc.Core" Version="2.29.0" />
     <PackageReference Include="Grpc.HealthCheck" Version="2.29.0" />
     <PackageReference Include="Grpc.Reflection" Version="2.29.0"/>