Switch font to Inter

We've long been discussing this within the GNOME design team, but recently realized there was no issue filed. So I'm filing it here to track it. 😄

Cantarell has served us well, but we've been wondering if it would be more beneficial to default to a more modern and well-maintained typeface, especially one that is actively developed to support new font features as they are developed. Inter fits this bill and is viewed favorably by some folks on the design team.

I also have an existing rapport with Rasmus, the designer and maintainer of Inter. He has indicated he would be happy to help us with any specific rendering questions and even participate in a technical discussion around the font rendering stack; he has extensive knowledge of the domain, having contributed to the rendering stack in Figma and Chromium.

Initiative: Teams/Design/initiatives#152