fix(form): avoid spreading key prop (#6776)

Before this change, React would throw a runtime error stating that `A props
object containing a "key" prop is being spread into JSX`. Now, the `key` is
passed to the JSX without using spread.

packages/sanity/src/core/form/studio/FormBuilder.tsx

@@ -153,7 +153,7 @@ export function FormBuilder(props: FormBuilderProps) {
     [Field],
   )
   const renderItem = useCallback(
-    (itemProps: Omit<ItemProps, 'renderDefault'>) => <Item {...itemProps} />,
+    ({key, ...itemProps}: Omit<ItemProps, 'renderDefault'>) => <Item key={key} {...itemProps} />,
     [Item],
   )
   const renderPreview = useCallback(