Flatpak release

[e-renna]

Pre-submission Checks

• I checked for similar issues, but could not find any. I also checked the closed issues. I could not contribute additional information to any existing issue.
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Feature description

It would be great if the team at ownCloud could release an official Flatpak of the ownCloud client.
Flatpak supports 38 Linux distributions, and has become the default way to install applications in atomic distributions such as Fedora Silverblue.

Proposed solution

Flatpak provides all the information to release a flatpak of an application in their documentation.

Alternative solutions you considered

No response

Additional context

This was previously raised in 2017 in #5716 regarding creating a Flatpak version of the NextCloud client.
However, 7 years have passed, this request is specific to ownCloud client (not NextCloud), and Flatpak popularity has increased. The distributions that support Flatpak have also increased, meaning that with a single release ownCloud would provide their client to 38 different distributions.

[TheOneRing]

We provide official AppImages, a community provided Flatpack would be more than welcome.

[TorrSamaho]

We provide official AppImages, a community provided Flatpack would be more than welcome.

I gave this a try and reworked the flatpak manifest for the Nextcloud client to build the ownCloud client instead: https://github.com/TorrSamaho/com.owncloud.desktopclient.owncloud

At least at first glance, the client GUI seems to work fine (tested under openSUSE Aeon).

[iasdeoupxe]

Follow-up available in #11989