Fixes #3484 (V1) - Updates SDK version to dotnet8, adds netstandard2.0 and net8.0 TargetFrameworks

[tig]

Fixes

• Fixes .NET 8 support for v1.xxx #3484

Todos

• Changes global.config to use dotnet8
• Adds netstandard2.0 to TFMs
• Adds net8.0 to TFMs
• Fixes warning in Clipboard.cs
• Updates nuget packages

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[dodexahedron]

I would suggest adding, not replacing, the target for net8.0, because .net 7 projects will grab the netstandard version if there's not a package targeting 7.

[dodexahedron]

And actually, unless it causes build errors, dropping 2.1 to 2.0 will enable framework projects to load the netstandard version as well. 2.1 was for Mono and added some goodies, but didn't get backported to .net framework.

If that does cause build errors, then of course leave 2.1 there for Mono peeps.

But we probably shouldn't be breaking compatibility with existing systems for any changes made to v1 til it's fully retired.

And adding multiple TFMs, if not build-breaking, costs us nothing. If an actual runtime issue crops up, we get to just say "sorry - not supported on EoL SDKs. Use at your own risk."