Bump button thin variants and icon buttons to 24px

[scurker]

WCAG 2.2 AA guidelines will now require target sizing to be a minimum of 24px: https://www.w3.org/WAI/standards-guidelines/wcag/new-in-22/

Target size (minimum): Renamed (from Pointer target Spacing), reduced the core size requirement to 24px, and simplified the criterion text.

Impacts

• Button (thin variant)
• IconButton (both height/width should match minimum target size)

[okry]

Pending official release of WCAG 2.2 guidelines.

[scurker]

We need to match whatever UXpin is here, it looks like uxpin has the target size > 24px

[scurker]

This is done.