Use netcoreapp3.1 instead of netcoreapp2.1

Author: FreePhoenix888

csharp/Platform.Random.Tests/Platform.Random.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net472;netcoreapp2.1;netcoreapp3.1;net5;net6</TargetFrameworks>
+    <TargetFrameworks>net472;netcoreapp3.1;netcoreapp3.1;net5;net6</TargetFrameworks>
     <IsPackable>false</IsPackable>
     <LangVersion>latest</LangVersion>
     <Nullable>enable</Nullable>