fix(dropdown): Use $scopedSlots instead of $slots

What does this MR do?

While $slots and $scopedSlots aren't exactly equivalent, there is a bug when sometimes the $slots object is empty even though the slot is provided. This usually happens when the component is re-rendered. This causes all slots in the dropdown component except default to not render at all in some ocassions.

This would fix the bug gitlab#390824 (closed).

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The possible caveat as per e766080e is that slot could be rendered even if it is empty. From a quick scan of all gl-dropdowns in use in gitlab, I found out that usually we have a v-if on the template, but the slot contents are always defined.

Does this MR meet the acceptance criteria?

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