Update to .NET 6

[daveaglick]

With .NET Core 3.1 going out of support, the time has finally come to update the Statiq target to .NET 6. This is also pushed a little bit by ongoing Buildalyzer updates to target .NET 6 which we need to be able to consume: Buildalyzer/Buildalyzer#221.

[phmatray]

Would it be possible to adopt the C# 11 standard?
.NET6 is a relevant choice because it is an LTS but .NET7 brings some improvements in the area of code generation.

[daveaglick]

There's certainly no reason your own consuming application couldn't target .NET 7, even if the Statiq dependencies are targeting .NET 6. Especially if what you're after is better codegen scenarios since those are mostly driven by the compilation process.