Target .NET 6, use `[SkipLocalsInit]` and remove Fody.

[teo-tsirpanis]

.NET 5 has introduced System.Runtime.CompilerServices.SkipLocalsInitAttribute that signals Roslyn to disable zero-initialization of variables. I bumped the target framework to .NET 6 to take advantage of this feature (hope there's no problem with that), and replaced Fody LocalsInit weaver with this attribute.

I also removed some obsolete analyzer packages, they are now built-in in the SDK.