Migrate to .NET 8

[siriak]

I propose to migrate to .NET 8 to use the latest features of .NET ecosystem.
I think we can do it in a few PRs to make them smaller and easier to review. One would just update .NET version and CI scripts and following would update NuGet packages and codebase.
Feel free to comment if you want to do it.

• SDK and build scripts
• File-scoped namespaces
• Implicit usings
• Nullable (if it's not enabled)
• Update dependencies

[gmottajr]

Hi Andrii,
I would like pick this one up, could you please assign it to me?
Any important details you can provide regarding this task would be greatly appreciated.
I will start to work on it as soon as I get a green light from you.
Thank you very much!

[siriak]

Sure, you can start. I have added a list of what needs to be done to the description of this issue. Just please make PRs small if possible so that it's not all in one giant PR.

[siriak]

Done, good job everybody! We will track #441 separately because it's an issue from .NET 6.