Update to Serilog 3 and update TFMs

[Numpsy]

Usual question about TFMs to target that pop up in all the repos - should it be a complete match for Serilog 3.0, or a smaller set of what's needed?

In this draft:

Both Framework 4.6.2 and 4.7.1 to create an explicit reference to the 4.7.1 version of the core lib that doesn't depend on the System.ValueTuple nuget package

.NET 6 as the oldest supported .NET Core version, and to explicity use the .NET Core versions of the Serilog core lib (avoids the dependencies on System.Diagnostics.DiagnosticSource in the latest versions

.NET Standard 2.0 for everything else.

So question - is there a need for .NET Standard 2.1 / NET 5.0/7.0 etc?

Note: The TFMs in the unit test project are just set to match the core Serilog lib

[Numpsy]

Related note: can see about doing the same change to the process and thread enrichers after the TFMs are confirmed, so they're all consisteny

[bartelink]

My opinion (but backed by some research, linked below)
Keep net462/net471 if they are not causing pain.
NS2.0 is good to keep for lots of reasons
Address core by supporting net6.0 unless there is something that can only be achieved by targeting a specific later TFM
Drop net5.0, NS21; don't add 7.0 (See also serilog/serilog#1966)

[Numpsy]

I'll close this in favour of #63

[nblumhardt]

Sorry @Numpsy, not sure how this slipped off the radar - I might be tracking a few too many things 😅