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.

[github-actions]

Everyone contributing to this PR have now signed the CLA. Thanks!

[j-j-gajjar]

I have signed the Canonical CLA. Please proceed with the review of my contribution. Thank you!

[Feichtmeier]

Hello everyone @anasereijo @BLKKKBVSIK 👋

This icon does not come out of nowhere.

In Yaru we have developed a free desktop icon theme with a very specific style guideline
Here is the source of the original icon

Extracted PNG:

Here are the guidelines for creating freedesktop Yaru icons:

https://github.com/ubuntu/yaru/wiki/%234-Help-for-creating-yaru-icons

Generally for this one and all future full color app icons please follow the yaru icon guidelines. Best regards, the Yaru Team

Edit: and here is a collection of all full color icon sources
https://github.com/ubuntu/yaru/tree/master/icons/src/fullcolor

here are the symbolic icon sources
https://github.com/ubuntu/yaru/tree/master/icons/src/scalable