cross target the samples to be sure it ran for both netcoreapp2.1 and 3.1

Author: tebeco

test/Serilog.AspNetCore.Tests/Serilog.AspNetCore.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp2.0</TargetFrameworks>
+    <TargetFrameworks>netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
     <AssemblyName>Serilog.AspNetCore.Tests</AssemblyName>
     <AssemblyOriginatorKeyFile>../../assets/Serilog.snk</AssemblyOriginatorKeyFile>
     <SignAssembly>true</SignAssembly>