Bump Google.Protobuf in /examples/csharp/HelloworldLegacyCsproj/Greeter (#28850)

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) from 3.8.0 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/v3.8.0...v3.15.0)

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

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examples/csharp/HelloworldLegacyCsproj/Greeter/packages.config

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
-  <package id="Google.Protobuf" version="3.8.0" targetFramework="net45" />
+  <package id="Google.Protobuf" version="3.15.0" targetFramework="net45" />
   <package id="Grpc" version="2.23.0" targetFramework="net45" />
   <package id="Grpc.Core" version="2.23.0" targetFramework="net45" />
   <package id="Grpc.Core.Api" version="2.23.0" targetFramework="net45" />