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.xUnit/GildedRoseTests/ApprovalTest.cs

@@ -1,5 +1,4 @@
-
-using GildedRoseKata;
+using GildedRoseKata;
 
 using System;
 using System.IO;
@@ -10,21 +9,20 @@ using VerifyXunit;
 
 using Xunit;
 
-namespace GildedRoseTests
+namespace GildedRoseTests;
+
+public class ApprovalTest
 {
-    public class ApprovalTest
+    [Fact]
+    public Task ThirtyDays()
     {
-        [Fact]
-        public Task ThirtyDays()
-        {
-            var fakeoutput = new StringBuilder();
-            Console.SetOut(new StringWriter(fakeoutput));
-            Console.SetIn(new StringReader("a\n"));
+        var fakeoutput = new StringBuilder();
+        Console.SetOut(new StringWriter(fakeoutput));
+        Console.SetIn(new StringReader($"a{Environment.NewLine}"));
 
-            Program.Main(new string[] { "30" });
-            var output = fakeoutput.ToString();
+        Program.Main(new string[] { "30" });
+        var output = fakeoutput.ToString();
 
-            return Verifier.Verify(output);
-        }
+        return Verifier.Verify(output);
     }
-}
+}