Stepper - code cleanup part 2

Follow on to https://github.com/grommet/grommet/issues/8076

Review Stepper code and determine if additional cleanups are needed.

Some areas I noticed:

• In StyledStepper there are some fallback tshirt sizes and pixel values, wondering if we should remove those
• In StyledIndicator we have defaultBackgroundHoverColor set to color-mix(in srgb, ${borderColor} 80%, black) and defaultColorHoverColor set to color-mix(in srgb, ${color} 80%, black), just want to confirm this is the desired behavior