Do not create request builder classes in parallel

This causes quite severe corruption of the CodeModel. Many threads
concurrently start creating the same models and start adding properties.
The result is undefined for larger models and will be different on every
run.

This probably fixes (or at least improves the situation for) microsoft#2442.

src/Kiota.Builder/KiotaBuilder.cs

@@ -702,15 +702,15 @@ private void CreateRequestBuilderClass(CodeNamespace currentNamespace, OpenApiUr
         CreateUrlManagement(codeClass, currentNode, isApiClientClass);
 
         if (rootNamespace != null)
-            Parallel.ForEach(currentNode.Children.Values, childNode =>
+            foreach (var childNode in currentNode.Children.Values)
             {
                 if (childNode.GetNodeNamespaceFromPath(config.ClientNamespaceName) is string targetNamespaceName &&
                     !string.IsNullOrEmpty(targetNamespaceName))
                 {
                     var targetNamespace = rootNamespace.FindOrAddNamespace(targetNamespaceName);
                     CreateRequestBuilderClass(targetNamespace, childNode, rootNode);
                 }
-            });
+            };
     }
     private static void CreateMethod(string propIdentifier, string propType, CodeClass codeClass, OpenApiUrlTreeNode currentNode)
     {