Upgrade to .NET 9 ++

[IEvangelist]

In addition to upgrading to .NET 9.0 this PR includes:

• Added namespaces to test classes.
• Added triple slash comments for all public APIs.
• Remove redundant NuGet packages, since Microsoft.Kiota.Bundle is used.
• Fix the README.md.
• Following naming rules for const variables/members.
• Add missing using statements in the GetGitHubAccessTokenAsync impl.
• Fix typo in comments.
• Readability issues addressed.
• Add missing access modifiers.
• Use existing primary project MSBuild copyright logic to dynamically get date time year.
  • $([System.DateTime]::Now.ToString('yyyy'))
• Move some classes and interfaces into their own file (improved searchability).
• Common/modern C# idioms:
  • Prefer primary constructors.
  • Prefer new().
  • Pattern matching.
  • Prefer collection expressions.
• Mark some classes as sealed as they do not seem to be designed for in inheritance.

[!NOTE] We might want to consider future work for using ILogger<T>, IHttpClientFactory, and maybe the built-in rate limiter.

Resolves #137

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Before the change?

After the change?

Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes
• [x] No

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[github-actions[bot]]

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