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
• No

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[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labeled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[nickfloyd]

Hey @IEvangelist Apologies for the delay in getting to this. I love these changes but I realized that there is an aspect here that we have failed to update regarding contributions. Since this SDK is generated - all of the prime source (the code that should be changed) exists here in the generator repo.

We left PRs unlocked in this repo because the generator creates PRs any time anything changes here or when there are new/updated OpenAPI definitions.

I hate to ask but would you mind migrating your changes to the generator and PR there? If not, no worries, we'll eventually get to it - we've just had a few priorities shift that has slowed our ability to get to all of the requests we get.

Thank you so much for the contributions and the continued work to make all of this better!