Serilog dropped cross-build for netcoreapp

[trejjam]

Description
A dependency resolution produces a lot of warnings.

....csproj : warning NU1603: Microsoft.Extensions.Configuration 2.1.1 depends on Microsoft.Extensions.Configuration.Abstractions (>= 2.1.1) but Microsoft.Extensions.Configuration.Abstractions 2.1.1 was not found. An approximate best match of Microsoft.Extensions.Configuration.Abstractions 3.0.0 was resolved.
....csproj : warning NU1603: Microsoft.Extensions.Hosting.Abstractions 2.1.0 depends on Microsoft.Extensions.Configuration.Abstractions (>= 2.1.0) but Microsoft.Extensions.Configuration.Abstractions 2.1.0 was not found. An approximate best match of Microsoft.Extensions.Configuration.Abstractions 3.0.0 was resolved.
....csproj : warning NU1603: Microsoft.Extensions.Hosting.Abstractions 2.2.0 depends on Microsoft.Extensions.Configuration.Abstractions (>= 2.2.0) but Microsoft.Extensions.Configuration.Abstractions 2.2.0 was not found. An approximate best match of Microsoft.Extensions.Configuration.Abstractions 3.0.0 was resolved.
....csproj : warning NU1603: Microsoft.Extensions.Options.ConfigurationExtensions 2.1.0 depends on Microsoft.Extensions.Configuration.Abstractions (>= 2.1.0) but Microsoft.Extensions.Configuration.Abstractions 2.1.0 was not found. An approximate best match of Microsoft.Extensions.Configuration.Abstractions 3.0.0 was resolved.
....csproj : warning NU1603: Microsoft.Extensions.Options.ConfigurationExtensions 2.0.0 depends on Microsoft.Extensions.Configuration.Binder (>= 2.0.0) but Microsoft.Extensions.Configuration.Binder 2.0.0 was not found. An approximate best match of Microsoft.Extensions.Configuration.Binder 2.0.1 was resolved.
....csproj : warning NU1603: Microsoft.Extensions.Options.ConfigurationExtensions 2.1.0 depends on Microsoft.Extensions.Configuration.Binder (>= 2.1.0) but Microsoft.Extensions.Configuration.Binder 2.1.0 was not found. An approximate best match of Microsoft.Extensions.Configuration.Binder 2.1.1 was resolved.
....csproj : warning NU1603: Microsoft.Extensions.Logging 3.0.0 depends on Microsoft.Extensions.Configuration.Binder (>= 3.0.0) but Microsoft.Extensions.Configuration.Binder 3.0.0 was not found. An approximate best match of Microsoft.Extensions.Configuration.Binder 3.0.1 was resolved.

What happens?/How to reproduce
Require Serilog.AspNetCore@4.0.0 -> it dropped conditional build for netcoreapp3.1 (and replaced it with netstandard2.1 - which is not the same). This adds a dependency on Microsoft.AspNetCore.Hosting.Abstractions which is a package from netcoreapp2.0.

Expected behavior
For netcoreapp3.0 and higher do not depend on Microsoft.AspNetCore.*@2.x packages.

Additional context
I can prepare a PR, if you like.

[trejjam]

The original PR with conditional build: #165

[nblumhardt]

Thanks for the heads-up - definitely open to a PR reinstating this target, if it's still a problem out there 👍