Upgrade to .NET 8, upgrade csharpier and lots of pkgs, fix tests and remove Roslyn 3.8/4.0 support, enable central package management

[martinothamar]

Todo

• [x] Verify samples still work
• [x] Update benchmarks and benchmark dependencies
• [x] Drop Roslyn 4.0 as well? Just keep 4.1. Would simplify things
• [x] Consolidate test projects? There's a lot of projects
• [x] Enabled Central Package Management (for everything except samples projects)

[TimothyMakkison]

Consolidate test projects? There's a lot of projects

The current test system is a little clunky, creating a whole new solution for a test case is annoying.

I'd considered changing tests to use a helper class with a variety of default programs where you can add or set a certain part. Currently Mediator uses something like:

var source = await Fixture.SourceFromResourceFile("DuplicateHandlersProgram.cs");

Instead:

var source = await TestHelper.PingRequest.With(
"""
public sealed class DuplicatePingHandler : IRequestHandler<Ping, Pong>
{
    public ValueTask<Pong> Handle(Ping request, CancellationToken cancellationToken)
    {
        return new ValueTask<Pong>(new Pong(request.Id));
    }
}
""");

This system could be used for certain tests, I'm not sure about all of them.

[martinothamar]

I managed to remove a couple of projects both for test and src due to getting rid of everything but roslyn 4.1 stuff The latest .NET 6 SDK has is 4.3.1 so hopefully this is fine :smile: