[C#][aspnet] Deprecating aspnet5 for aspnetcore

[jimschubert]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch 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)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description

This moves previous aspnet5 functionality to aspnetcore, aspnet5 now
derives from the aspnetcore generator and logs a warning that it's
deprecated.

This will help resolve any confusion because ASP.NET 5 is terminology
referring to the project of around a year ago. ASP.NET Core 1.0 is the
new name for the technology, and further enhancements should target .NET
Core 1.0 rather than any old vNext or preview stuff.

See #3085

I ran the bash script, no code changed other than path renames.
This would be a breaking change if CI relies on the aspnet5 paths or files under ./bin (I didn't see any).

The aspnet5 generator still exists, and works as it did before... it just logs a warning now that you should use aspnetcore now.

If this gets merged, the Wiki at https://github.com/swagger-api/swagger-codegen/wiki/Server-stub-generator-HOWTO#aspnet-5-web-api should update from "ASP.NET 5 Web API" to "ASP.NET Core 1.0". I don't think "Web API" is a relevant term anymore. I'm pretty certain it's now considered "ASP.NET Core 1.0 MVC", but I think the MVC is pretty much implied now.

[wing328]

@jimschubert thanks for the PR. I've not setup any CI for aspnet5. I only setup appveyor to cover Petstore C# API client.