Refactoring c# projects (#544)

Small Refactoring c# projects, e.g.
* add days
* remove StartupObject
* Fix Approval tests for NUnit
* Use Environment.NewLine in Xunit tests
* Added + fixed README.md
* Removed global.json

csharp.NUnit/GildedRoseTests/ApprovalTest.cs

@@ -1,10 +1,14 @@
+using GildedRoseKata;
+
 using System;
 using System.IO;
 using System.Text;
 using System.Threading.Tasks;
-using NUnit.Framework;
+
 using VerifyNUnit;
 
+using NUnit.Framework;
+
 namespace GildedRoseTests;
 
 public class ApprovalTest
@@ -16,7 +20,7 @@ public class ApprovalTest
         Console.SetOut(new StringWriter(fakeOutput));
         Console.SetIn(new StringReader($"a{Environment.NewLine}"));
 
-        TextTestFixture.Main(new string[] { "30" });
+        Program.Main(new string[] { "30" });
         var output = fakeOutput.ToString();
 
         return Verifier.Verify(output);