Docs: mention removal of util.js in migration guide (#41187)

Author: julien-deramond

site/content/docs/5.3/migration.md

@@ -736,3 +736,5 @@ Want more information? [Read the v5.1.0 blog post.](https://blog.getbootstrap.co
 - The default value for the `fallbackPlacements` is changed to `['top', 'right', 'bottom', 'left']` for better placement of Popper elements. **Applies to dropdowns, popovers, and tooltips.**
 
 - Removed underscore from public static methods like `_getInstance()` → `getInstance()`.
+
+- Removed `util.js`, with its functionality now integrated into individual plugins. If you previously included `util.js` manually, you can safely remove it, as it is no longer needed. Each plugin now contains only the utilities it requires, enhancing modularity and reducing dependencies.