Add the hack to compile Net6 from Net7

csharp/Lib/Utils/HackToBuildNet6FromNet7.cs

@@ -0,0 +1,15 @@
+// TODO remove for .Net 7
+namespace System.Runtime.CompilerServices
+{
+    public class RequiredMemberAttribute : Attribute { }
+    public class CompilerFeatureRequiredAttribute : Attribute
+    {
+        public CompilerFeatureRequiredAttribute(string name) { }
+    }
+
+}
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    public class SetsRequiredMembersAttribute : Attribute { }
+}