[BUG] Document values for aspnetcore buildTarget

Created by: hauntingEcho

Bug Report Checklist

• [ N/A ] Have you provided a full/minimal spec to reproduce the issue?
• [ N/A ] Have you validated the input using an OpenAPI validator (example)?
• [ X ] Have you tested with the latest master to confirm the issue still exists?
• [ X ] Have you searched for related issues/PRs?
• [ X ] What's the actual output vs expected output?
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Description

The documentation for aspnetcore doesn't list valid values for buildTarget, which are "library" and "program".

openapi-generator version

public documentation site and the master branch's docs folder