Add relevant appstream data to RPM repository

[amethystgosling163]

I'm using the Fedora 40 repository, but I'm not seeing an app page for Mullvad. It's important because, with it:
There can an app page for Mullvad in appstream app centers (mintInstall, KDE Discover, GNOME Software) that shows when the repository is enabled. The benefit for providing this page is that the user can uninstall Mullvad graphically right from this page.

Secondly, the browser shows up / is identified as an app in the updates tab and isn't grouped under system updates (it's a separate dialog in GNOME Software that can be accessed from the update tab). This is important so that the user knows from their software center's updates page that Mullvad browser has got an update.

Finally, the OS notifies you to update as soon as possible when Mullvad has an update because it's an important security update.