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/DictionaryCacheRefresherNotification.cs

@@ -0,0 +1,11 @@
+using Umbraco.Cms.Core.Sync;
+
+namespace Umbraco.Cms.Core.Notifications
+{
+    public class DictionaryCacheRefresherNotification : CacheRefresherNotification
+    {
+        public DictionaryCacheRefresherNotification(object messageObject, MessageType messageType) : base(messageObject, messageType)
+        {
+        }
+    }
+}