build(deps): bump Google.Protobuf in /demos/net5/GrpcDemo

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.1 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.1...v3.15.6)

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

demos/net5/GrpcDemo/GrpcClient/GrpcClient.csproj

@@ -6,7 +6,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.15.1" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.6" />
     <PackageReference Include="Grpc.Net.ClientFactory" Version="2.35.0" />
     <PackageReference Include="Grpc.Tools" Version="2.35.0">
       <PrivateAssets>all</PrivateAssets>

demos/net5/GrpcDemo/GrpcClientHosted/GrpcClientHosted.csproj

@@ -16,7 +16,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.15.1" />
+    <PackageReference Include="Google.Protobuf" Version="3.15.6" />
     <PackageReference Include="Grpc.Net.ClientFactory" Version="2.35.0" />
     <PackageReference Include="Grpc.Tools" Version="2.35.0">
       <PrivateAssets>all</PrivateAssets>