Updated to .NET Core 2.1

[knom]

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 and ./bin/security/{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: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Updated ASPNET Core Generator to support .NET Core 2.1.
As per https://docs.microsoft.com/en-us/dotnet/core/migration/20-21

@jimschubert @wing328

[knom]

This is solving #8761

[knom]

@wing328 can you merge this?

[jimschubert]

@knom we've moved to our own fork (https://github.com/OpenAPITools/openapi-generator), and neither of us have commit access to this project.

openapi-generator had a similar PR a couple months ago. See OpenAPITools/openapi-generator#1008

[HugoMario]

already covered