Update app-center icon with new images

[j-j-gajjar]

This PR adds two new images, app-center.svg and 2.0x/app-center.png, to the repository. While these images are not currently in use, they are included to provide updated assets for potential future use. These images were provided by @BLKKKBVSIK and are intended to enhance the visual representation of the app-center icon. By incorporating these images into the repository now, we ensure that they are readily available for any future updates or implementations. This proactive approach to asset management helps maintain consistency and quality across the Ubuntu app ecosystem.