Port changes from https://github.com/octokit/dotnet-sdk/pull/138

[IEvangelist]

Port changes from octokit/dotnet-sdk#138

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

• Migrate from .sln to .slnx.
• 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.

[IEvangelist]

/CC @nickfloyd