use Nullable packages instead of attribute

since it also gives us the other nullable attributes

Author: SimonCropp

src/Serilog.Expressions/Compatibility/NotNullAttributes.cs

@@ -24,6 +24,7 @@
 
   <ItemGroup>
     <PackageReference Include="Serilog" Version="2.10.0" />
+    <PackageReference Include="Nullable" Version="1.3.0" Condition="$(TargetFramework) == 'netstandard2.0'" />
   </ItemGroup>
 
   <ItemGroup>