Super sidebar GlDisclosureDropdown update expanded styles selectors

Discussion in !112880 (comment 1305913287)

One other thing I noticed is I'm not seeing the "active" state on the button for the MR dropdown when it is open (like in this design). I think these styles are meant to account for it, though doesn't seem as though it's being applied.

Improved semantics in gitlab-ui!3279 (merged) can be reflected in super sidebar CSS selectors.