Add a .NET 8.0 target

[manfred-brands]

Fixes #4890
Works toward #4857

This adds the .NET 8.0 target. So that if users that are not allowed to use end-of-life frameworks due to lack of security updates can the .NET8.0 binaries.

[mikkelbu]

Looks good - only comments and questions from me 👍

[mikkelbu]

Perhaps we should rather remove this ?

[mikkelbu]

Just suggestions. Should we move the usings outside the namespace? (and perhaps move System.Runtime.Serialization down to the sole purpose, so we avoid having two !NET8_0_OR_GREATER blocks ?

EDIT: I can see that we do it differently in many of the ...Exception files, so I'm also happy with the current solution (to keep the fix as small as possible)

[mikkelbu]

Why is this need now ?

[mikkelbu]

Doesn't #if !NET6_0_OR_GREATER imply #if !NET8_0_OR_GREATER ?
Or is this to to anticipate when we remove the .NET6 target ?

[OsirisTerje]

We need to add the .net 8 bits to the nunit.nuspec file, otherwise the bits don't go into the nupkg.

[manfred-brands]

We need to add the .net 8 bits to the nunit.nuspec file, otherwise the bits don't go into the nupkg.

Thanks @OsirisTerje

I forgot and to ensure nobody will forget it again, I added a nunit test to verify proper file references in .nuspec files.
Similar to the one you added for dependencies.

The bulk of the size is the 3 copies of the .xml files 1.2MB per target.
They are 99% the same with a few TFM specific APIs.
I tired to put only one of the in lib/any but it then doesn't seem to be picked up.
Searching the net didn't result in any solutions.

[OsirisTerje]

Lgtm, @manfred-brands : can we merge ?