Bump GraphQL.SystemTextJson from 3.0.0.2026 to 3.1.3 (#85)

Bumps [GraphQL.SystemTextJson](https://github.com/graphql-dotnet/graphql-dotnet) from 3.0.0.2026 to 3.1.3.
- [Release notes](https://github.com/graphql-dotnet/graphql-dotnet/releases)
- [Commits](https://github.com/graphql-dotnet/graphql-dotnet/commits/3.1.3)

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src/BasicSample/BasicSample.csproj

@@ -11,7 +11,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="GraphQL.SystemTextJson" Version="3.0.0.2026" />
+    <PackageReference Include="GraphQL.SystemTextJson" Version="3.1.3" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="3.1.9" />
   </ItemGroup>