Add icon only variant to back to top in Figma

[ad9242]

Summary

Work has been done to add a new icon only variant of the back to top component. This was completed in the developed components under https://github.com/mi6/ic-ui-kit/issues/1849

An update to Figma is required to ensure equivalent functionality is available

The terminology used in the build component is variant with a value of icon or default. If this doesn't seem appropriate, we could raise a ticket to revise this

📝 Acceptance Criteria

If relevant, describe in full detail the different interactions and edge cases that the component or patterns needs to fulfil.

Given a back to top component exists When using the component Then an option to use an icon only variant, without a label, exists

Guidance

will be covered by #958