Change target frameworks to fix warnings and allow updating dependencies

[Morilli]

Currently, BizHawk.BizInvoke target netframework 4.8 because it uses certain APIs that aren't available under netstandard2.0. Other projects depending on BizHawk.BizInvoke target netstandard2.0 though, which generates NU1702, which is currently suppressed.

If we want to update System.Memory (which is required at least for other dependencies), we start getting tons of MSB3277 warnings because the framework version of the System.Memory dll has a higher assembly version than the netstandard one, resulting in unresolvable conflicts on build. Those are of course not fatal because the API surface is the same, so it could probably also be ignored.

However, considering that we're getting warnings that need to be suppressed simply because we're doing questionable things with target framework and dependencies I'd argue adjusting the TargetFramework property is the more correct resolution here, unless someone can convince me that targeting netstandard instead of net framework has any advantages.

[YoshiRulz]

I think the only thing that having main solution libs target netstandard2.0 does is that it prevents accidentally using APIs which wouldn't be available in .NET Core. We could use Microsoft.CodeAnalysis.BannedApiAnalyzers to block those instead.

[Morilli]

Are there any relevant apis that only apply to net framework and not .NET (core)? If there are it's probably fine to disallow them, I'm not against using netstandard in places where it doesn't result in issues.

[SuuperW]

Looks good to me.