[ic-toast]: Developed toast has no outer border unlike the designed version

[GCHQ-Developer-112]

Summary of the bug

In designed version of ic-toast, there is a border around the toast that matches the colour of the variant. This does not match up with the current developed version of ic-toast which has no outer border.

🪜 How to reproduce

View "Success Toast" story in Storybook and compare with the either the screenshot below or Figma

📸 Screenshots or code

🧐 Expected behaviour

There should be an outer border around the current developed version of ic-toast so that it reflects the designed version

🚨 Urgency (low, medium or high)

Medium

[MI6-255]

https://github.com/mi6/ic-ui-kit/issues/1224