[BUG] [csharp-netcore]: Failing to load the project

Created by: saigiridhar21

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

• Visual studio is failing to load the SDK client project which is generated for netstandard2.0.
• netcoreapp's test project is missing xunit related dependencies.
• When no targetFramework is provided, openapi-generator is picking v4.6.1 as default. It should be netstandard2.0.
• Removed the hard-coded paths of NETStandard.Library.2.0.3 in the csproj files.

openapi-generator version

4.1.0

OpenAPI declaration file content or url

Command line used for generation

Use PetStore project to check.

Steps to reproduce

Open Petstore project(generated for netstandard2.0) on visual studio and it will fail to load the client project stating that it needs migration.

Related issues/PRs

Suggest a fix

• Fix the Project.mustache as per the standards of netstandard.
• Fix the netcoreapp's test project to include xunit related dependencies.
• Update the default value to netstandard2.0 when user doesn't provide targetFramework option.