Fix cursor on disabled GlDisclosureDropdown custom toggles so it matches
the `not-allowed` cursor of other disabled buttons, instead of showing a
pointer.

Fix disabled dropdown toggle cursor

See merge request !5960



Merged-by: Thomas Hutterer <thutterer@gitlab.com>
Approved-by: Thomas Hutterer <thutterer@gitlab.com>
Co-authored-by: Chad Lavimoniere <clavimoniere@gitlab.com>