Update the navigation sketches to use the correct terms

According to the decision meeting we had on 19 march, se agree that "task" is the new concept for the new navigation and overview page. We also discussed other terms pertaining to navigation, for example page group. Decisions and most definitions are are recorded here:

https://www.figma.com/design/VAvGOqkMhKM8HL8h4xBeDH7b/Re-design-Altinn-Studio?node-id=16774-51283&t=djT6q3BVBkzQI0hl-4

