Update icon, font size, color for Learning Duration

[ryelle]

Fixes #2627 — This updates the icon for the Learning Duration block to use the SVG from figma, instead of a dashicon. It also updates the style attributes on each instance of the block so that it matches the "Lesson Count" block, which inherits text color and uses extra-small font. This matches the figma, which is 12px default text color (charcoal-1).

Before	After

To test

• Add durations to some courses. This block is also used on lessons, though none on production seem to have duration; but the style update has been done for lessons as well.
• Everywhere the duration appears, it should be 12px font size, charcoal-1 color, and the icon should be an SVG (the SVG has a thinner circle border around it)

[adamwoodnz]

LGTM 🚢