Form Select > Align styles with dropdowns

Currently selects and dropdowns' toggles look slightly different:

Dropdown (listobox)	Select

Because those components can sometimes be used in the same context (for example, the user preferences form combines listboxes and selects), this styling discrepancy can look a bit odd.

Let's look into making them look bit more alike.

Re gitlab-org/gitlab!124233 (comment 1438934487)