V9: Place notifications in the same namespace (#10231)

* Gather all notifications in Umbraco.Cms.Core.Notifications

* Rename notifications to match convention

* Move and rename missed notifications

* Move the three remaining public notification into Umbraco.Cms.Core.Notifications

src/Umbraco.Core/Notifications/UserGroupWithUsersSavingNotification.cs

@@ -0,0 +1,19 @@
+// Copyright (c) Umbraco.
+// See LICENSE for more details.
+
+using System.Collections.Generic;
+using Umbraco.Cms.Core.Events;
+
+namespace Umbraco.Cms.Core.Notifications
+{
+    public sealed class UserGroupWithUsersSavingNotification : SavingNotification<UserGroupWithUsers>
+    {
+        public UserGroupWithUsersSavingNotification(UserGroupWithUsers target, EventMessages messages) : base(target, messages)
+        {
+        }
+
+        public UserGroupWithUsersSavingNotification(IEnumerable<UserGroupWithUsers> target, EventMessages messages) : base(target, messages)
+        {
+        }
+    }
+}