Update serilog nuget package to 2.8.0 version

[vhatsura]

Update of Serilog package has two reasons:

1. Serilog 2.8.0 has separate target for .netstandard2.0;
2. Dependency graph was simplified

Serilog 2.6.0:

.NETStandard 1.3
Microsoft.CSharp (>= 4.0.1)
System.Collections (>= 4.0.11)
System.Collections.NonGeneric (>= 4.0.1)
System.Dynamic.Runtime (>= 4.0.11)
System.Globalization (>= 4.0.11)
System.Linq (>= 4.1.0)
System.Reflection (>= 4.1.0)
System.Reflection.Extensions (>= 4.0.1)
System.Runtime (>= 4.1.0)
System.Runtime.Extensions (>= 4.1.0)
System.Text.RegularExpressions (>= 4.1.0)
System.Threading (>= 4.0.11)

Serilog 2.8.0:

.NETStandard 1.3
NETStandard.Library (>= 1.6.1)
System.Collections.NonGeneric (>= 4.3.0)
.NETStandard 2.0
System.Collections.NonGeneric (>= 4.3.0)

[mivano]

Nice, thanks!