Build a package version for net461

[alastairs]

This will be resolved by NuGet for .NET Framework 4.6.1 upward, and is equivalent to the netstandard2.0 package version.

Fixes #123.

[MV10]

When I made the multi-target changes, I believe someone told me Serilog still has .NET 4.5.x users...

[alastairs]

Thanks @MV10. I don't believe my proposed changes remove support for those users?

[MV10]

Oops, you're right, sorry about that.

Did you see the NuGet issue I linked to in #123? I don't use .NET Framework at all now, so I can't easily check this, but I'm wondering if this change prevents .NET Framework apps from using the old config if that is a requirement.

[alastairs]

Hi, yes, I did. I wonder if there's a difference in the way the projects are built. We are using the .NET SDK across all of our projects (mostly targeting .netcoreapp2.0, but some target net471 as per #123), and we use the new-style .csproj files with PackageReferences everywhere. We do definitely the see the opposite of what is reported in that linked issue. I'm not sure, but am wondering if the project in the linked issue is an old-style .NET Framework project.

[MV10]

I hadn't realized how far the latest VS goes to bury .NET Framework, it took me quite awhile to even figure out how to target it, but I think it tests out OK, at least building a NuGet package locally (I can't be 100% sure it'll match the published Serilog packaging, I can't see how that is set up).

Incidentally Nick commented on one of my PRs that we may need to consider dropping support for older versions, but it doesn't look like that day is upon us yet.

Looks good to me, PR merge in 3, 2, 1...

[alastairs]

Brill, thanks @MV10!