Bump Google.Protobuf from 3.8.0 to 3.15.0 in /csharp_grpc_bench/Greet…

…er (LesnyRumcajs#192)

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](protocolbuffers/protobuf@v3.8.0...v3.15.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

csharp_grpc_bench/Greeter/Greeter.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.8.0" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.0" />
     <PackageReference Include="Grpc" Version="2.23.0" />
     <PackageReference Include="Grpc.Tools" Version="2.23.0" PrivateAssets="All" />
   </ItemGroup>