Update test projects to use "net8.0" as a moniker for testing

[zbalkan]

Since .NET 7.0 EOL, it's better to stick to LTS version that is till supported. I used LTS instead of LTS to minimize friction between new versions.

[ViktorHofer]

@jeffhandley @adamsitnik I think it would be good to consider this change for the next possible nuget.org release.

[zbalkan]

5 commits for this small change may look a bit messy. Should I squash and force-push or do you prefer a merge with squash after it is approved?

[jonsequitur]

Other than the package version change for System.Memory, these changes only affect tests. The System.CommandLine library already targets net8.0 and netstandard2.0.

[ViktorHofer]

Right but we shouldn't be depending on old / obsolete package dependencies, specifically System.Memory/4.5.4 and Newtonsoft.Json/13.0.2 in upcoming releases.

addressed

[Frulfump]

From PR description

LTS instead of LTS

?!
PR title is a bit confusing since the focus is the .NET version used in tests, and the other changes bump some packages not necessarily tied to .NET 8? And it show up as just that in the GitHub release description.

[adamsitnik]

From PR description

LTS instead of LTS

?! PR title is a bit confusing since the focus is the .NET version used in tests, and the other changes bump some packages not necessarily tied to .NET 8? And it show up as just that in the GitHub release description.

Good point, I've renamed it to "Update test projects to use "net8.0" as a moniker for testing "