feat(GlListbox,GlDisclosure): Update dropdown styles

What does this MR do?

Updates new dropdown styles according to the new designs - this would be Step 1 for #2125 (closed)

Overflow scrim (step 2) and arrow/tick (step 3) are out of the scope of this MR.

I've separated old and new dropdown styles so that dropdowns implemented with GlDropdown will look as before whereas GlCollapsibleListbox and GlDisclosureDropdown will be redesigned.

All new dropdown CSS classes are starting with gl-new-dropdown prefix but if you can think of a better naming - I'm open to suggestions.

👀 Check te latest Review App to see the changes

References #2125 (closed)