XWIKI-22916: The xclass migrator is modifying the cached document

(cherry picked from commit ba6d468c)

xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/internal/objects/classes/XClassMigratorListener.java

@@ -181,6 +181,9 @@ private void migrate(PropertyClass newPropertyClass, BaseProperty<?> newProperty
         if (!document.isNew()) {
             boolean modified = false;
 
+            // Avoid modifying the cached document
+            document = document.clone();
+
             for (BaseObject xobject : document.getXObjects(classReference)) {
                 if (xobject != null) {
                     BaseProperty<?> property = (BaseProperty<?>) xobject.getField(propertyReference.getName());