.NET 8 Console and Agent

[mlessmann]

• Use .NET 8 SDK in global.json
• Add .NET 8 target to nunit-console and nunit-engine-core
• Add .NET 8 and 7 target to nunit-agent
• Selectively disable NuGet audit to suppress errors when building for .NET Core 2.1
• Adjusted cake scripts

See #1374

[mlessmann]

@dotnet-policy-service agree company="dSPACE GmbH"

[OsirisTerje]

Looks good to me. I can't see anything that seems out of place.

[CharliePoole]

@OsirisTerje @mlessmann
I'm a bit confused, as I was about to add a .NET 8.0 agent for 3.18.

For some reason, this PR was merged into branch 3.15 and is also now in main, but I can't figure out how it got there.
It wasn't in release 3.17. I think this also applies to other work that @mlessmann did and I'd like to straighten it out. At a minimum, I'll create a "retroactive issue" for 3.18, so that the release notes show it as being added and @mlessmann is credited.

@mlessmann I think this also applies to some other commits you made, which I'm investigating. Can we set up a channel to communicate without using issue comments? My email is on my GitHub profile.