Loading indicator should always be the circular variant when at `icon` size

[GCHQ-Developer-847]

Summary of the bug

When the size prop on the loading indicator is set to icon, it should always show the circular variant and never the linear variant (this is the case in the Figma designs).

🪜 How to reproduce

1. Go to the story for the icon loading indicator and change the type in the DevTools to linear.
2. See that it tries to render a very small linear loading indicator e.g.:

🧐 Expected behaviour

As defined on Figma, the loading indicator should always display the circular variant when it is set to its icon size.

Additional info

It is probably worth adding a note to the description for the size prop to say that setting it to icon will always display the circular indicator.

[MI6-255]

Design thinks likely best to keep consistent with Button loading, so Figma needs updating

[mi6-577]

Duplicate issue to #1476