[C#][.NET Core] fix map instantiation in models extending Dictionary

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
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Description of the PR

Fix the following build error:

➜  OpenAPIClient git:(csharp-fix-map) ✗ dotnet build
Microsoft (R) Build Engine version 15.9.20+g88f5fadfbe for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 294.31 ms for /Users/williamcheng/Code/graphql-generator/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Org.OpenAPITools.csproj.
  Restore completed in 2.19 sec for /Users/williamcheng/Code/graphql-generator/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools.Test/Org.OpenAPITools.Test.csproj.
Model/AdditionalPropertiesObject.cs(33,74): error CS0305: Using the generic type 'Dictionary<TKey, TValue>' requires 2 type arguments [/Users/williamcheng/Code/graphql-generator/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Org.OpenAPITools.csproj]

Build FAILED.

Model/AdditionalPropertiesObject.cs(33,74): error CS0305: Using the generic type 'Dictionary<TKey, TValue>' requires 2 type arguments [/Users/williamcheng/Code/graphql-generator/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Org.OpenAPITools.csproj]
    0 Warning(s)
    1 Error(s)

Also added ./bin/csharp-netcore-petstore.sh script to bin/utils/ensure-up-to-date script