fix(GlDisclosureDropdown): Ensure default ID is unique

What does this MR do?

fix(GlDisclosureDropdown): Ensure default ID is unique

As it was, all instances of this component which don't have toggleId passed explicitly would share the same id, since the default was evaluated at script import time, rather than component instance initialisation.

This was initially implemented in !3254 (merged), then reverted in !3244 (comment 1259093614). This re-adds the change.

Does this MR meet the acceptance criteria?

Conformity

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• Added the ~"component:*" label(s) if applicable.

Security

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Accessibility

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