Deprecate or update `border.radius.small`

[JasonStoltz]

After updating border.radius.medium to 4px (PR), both border.radius.medium and border.radius.small are now set to 4px, making them redundant.

We will need to either deprecate and remove the border.radius.small option, or update to it to use a smaller value than 4px.

When we do this, please be sure to remove the note regarding this redundancy from https://eui.elastic.co/docs/getting-started/theming/tokens/borders/#radius.