fix(next): admin panel UI not rendering custom upload components (#9925)

### What?

Currently it is not possible to override the upload component.

### Why?

The ability to override the upload component is missing from
`renderDocumentSlots`.

### How?

Adding a check to include the custom upload component if it is
available.

This issue is holding me back from releasing a payload plugin.

Fixes #9591

packages/next/src/views/Document/renderDocumentSlots.tsx

@@ -115,6 +115,14 @@ export const renderDocumentSlots: (args: {
     }
   }
 
+  if (collectionConfig?.upload && collectionConfig?.admin?.components?.edit?.Upload) {
+    components.Upload = RenderServerComponent({
+      Component: collectionConfig.admin.components.edit.Upload,
+      importMap: req.payload.importMap,
+      serverProps,
+    })
+  }
+
   return components
 }