Bump Google.Protobuf

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) from 3.6.1 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/v3.6.1...v3.15.0)

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

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

Private/CloudStorePlayground/GrpcCopyTesting/ProtoGen/ProtoGen.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.6.1" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.0" />
     <PackageReference Include="Grpc" Version="1.18.0" />
     <PackageReference Include="Grpc.Tools" Version="1.18.0" />
     <Protobuf Include="Test.proto" />