fix: use __APPEND__ in adminPanel to not overwrite system items

Without __APPEND__, the itemList replaced all existing system admin
panel entries instead of adding to them.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

files/custom/Espo/Modules/DataMigration/Resources/metadata/app/adminPanel.json

@@ -1,6 +1,7 @@
 {
     "system": {
         "itemList": [
+            "__APPEND__",
             {
                 "url": "#Admin/dataMigration",
                 "label": "Data Migration",