GlDisclosureDropdown > Keep the focus on the toggle upon opening the dropdown

Currently, GlDisclosureDropdown handles the focus like GlCollapsibleListbox does: when the dropdown is opened, the focus is automatically moved from the toggle to the first focusable item within the menu. This is not desirable in this context as @jeldergl noted in gitlab!109246 (comment 1247662543):

for the + menu, I think it should remain focused until focus is moved into the panel. This is the correct behavior for a disclosure. The behavior I'm seeing looks more like a collapsible listbox, in which case, the focus would move to the options and making use of a roving tab index.

Let's disable this behavior in GlDisclosureDropdown.