gstreamer 2.22, webkitgtk 2.40.0, qt 5.15.8 and ffmpeg 6 on staging

[Maxim Cournoyer]

Hi,

I've updated the following dependencies in a group (to try to make
things a bit more efficient) on the staging branch; the motivation
originally stemmed from the latest Jami now requiring FFmpeg 6.

It'd be useful if people tested it by reconfiguring their systems with
it or updating their profiles, and report any issues, as I'd like to
merge this branch into master in about a week time, if there are no
blockers.

-- 
Thanks,
Maxim