Responsive improvements to submenus in the adminbar:

- Remove .ab-sub-wrapper rules in the 782px media query, as they're duplicates of the rules set on line 161.
- Reposition the arrows for submenus at responsive sizes where the menus are taller.
- Set the submenu position to static at smartphone sizes, so it's not positioned outside of its parent (and thus offscreen).

Fixes #26720, props undergroundnetwork, iammattthomas.



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