Use QDesktopServices on Linux too with Qt 6, which fixes opening URLs

[matterhorn103]

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[matterhorn103]

The links to the website, forums, GitHub bug reports etc. are broken in Qt6 builds currently. This fixes it.

I think the native builds are broken because the use of a single string with QProcess::execute() is deprecated. In theory it could be fixed just by breaking up the arguments into a list of strings.

However, QDesktopServices::openUrl() goes via the appropriate XDG portal on Linux, which means that it works even from a sandbox, so this works for the Flatpak (verified) and presumably will for the AppImage too.

(In general we should use the portals on Linux wherever possible, which basically means using QDesktopServices::openUrl() and the normal QFileDialog).

I suspect that the original issue that led to Linux being handled differently was due to the user not having xdg-desktop-portal set up and running correctly. But like I say, I can't work out when QDesktopServices started making use of the XDG portal, so for safety I kept the behaviour on Qt5 the same.