Update Linux installation method on PrusaSlicer installation page

[brtmax]

Is your feature request related to a problem? Please describe.
Installation instruction on the PrusaSlice website are not accurate anymore. Suggested Linux installation method on the official PrusaSlicer install page is not accurate anymore, since flathub is now the only official distribution channel. Installation via AppImage is still suggested on the installation page. People have mentioned it in the comments there before, but it hasn't been picked up yet.

https://help.prusa3d.com/article/install-prusaslicer_1903

Describe the solution you'd like
Update the Linux installation instructions. I'd be happy to provide an updated section

Describe how it would work
Be as descriptive as your skill level allows you.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or paint drawing about the feature request here.

https://help.prusa3d.com/article/install-prusaslicer_1903#linux
https://github.com/prusa3d/PrusaSlicer/releases/tag/version_2.9.0-alpha1 (bottom of the page)

Apologies if this is the wrong place to report this issue. Comments mentioning this on help.prusa3d.com are quite old, so I thought it's best to report it elsewhere too. If there is a better place for this, please refer me to it.