[BUG][csharp-netcore] Fix XML documentation warnings

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?
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Description

• Fix XML documentation warnings that are being shown while building the package.
• Fix the hardcoded PetApi class name in api.mustache.

openapi-generator version

4.1.0

OpenAPI declaration file content or url

Command line used for generation

Steps to reproduce

Open the Pet project using visual studio and try to build the package to check all the warnings.

Related issues/PRs

Suggest a fix

Fixing the documentation.