Update flathub.json

[Citrodata]

Add aarch64

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[hadess]

Appdata fixes were made in #14 so please drop those changes from your PR and rebase.

As far as the flathub.json is concerned, you can probably simply remove it, as i386 isn't a supported arch anymore (for apps at least), and x86_64 + aarch64 covers all the supported arches.

Note that I'm also updating the runtime here: #15
So it will need to build with that runtime version for me to merge your changes.

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[hadess]

Seeing as you closed the PR, I've made those changes in #16 instead