Use the new Flathub badges

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

I read https://fosstodon.org/@bragefuglseth/112753049810610017 and found out there is a new badge. I won’t have access to my computer for a week so opening this for now.

Describe the solution you'd like to see.

Simple one-line change in the README and potentially in the Wiki as well.