Migrate octokit.NET to the new generated SDK

[Cai1Hsu]

GitHub 正在迁移到 kiota 构建的 SDK，因此原有的 octokit 将不再积极维护。并且 octokit 不包含 Classroom 相关 API。详细见下文：

Hey @Cai1Hsu,

Thanks for reaching out. You are correct. Maintaining this SDK is a manual task, which is > why we've made the move to a generated SDK. You'll find that our new generated .NET SDK has more feature and almost 100% of all GitHub REST APIS.

While octokit.net is still maintained our focus has shifted to the new approach for exactly > the reason you mentioned above.

Going forward our community has been mostly driving the additions of APIs to this SDK while > we've been focused on building out the other.

Please feel free to add the APIs the this SDK or give the newer one a shot.

Originally posted by @nickfloyd in octokit/octokit.net#2967 (comment)

Task list:

• 删除 octokit.NET 引用
• 添加 GitHub.Octokit.SDK 引用
• 重新实现 octokit.NET 相关代码
• 重构已有代码以尽量复用 GitHub.Octokit.SDK 项目