Bug#966535: transition: pipewire 0.3

[Simon McVittie <smcv@debian.org>]

Control: tags -1 - moreinfo

I think we're ready for a transition slot. The plan is:

- Upload experimental pipewire (0.3.x) to unstable

- Deal with dependent packages:
  + gnome-remote-desktop
    * Remove from testing until we have a GNOME version that makes it
      practically useful (it is not currently usable, see #966521)
  + weston
    * Either upload experimental version to unstable, or disable
      pipewire integration in unstable (as was already done in experimental)
  + krfb
    * Maintainers say they will apply
      https://salsa.debian.org/qt-kde-team/kde/krfb/-/merge_requests/2 in
      unstable when the transition goes ahead, but do not want to upload it
      to experimental right now
    * Contingency plan: disable its pipewire support instead
  + xdg-desktop-portal-kde
    * The KDE team tell me the version in experimental is not yet suitable
      for unstable
    * Maintainers say they will apply
      https://salsa.debian.org/qt-kde-team/kde/xdg-desktop-portal-kde/-/merge_requests/5
      in unstable when the transition goes ahead
    * Contingency plan: disable its pipewire support instead

- (Transition is formally over at this point)

- Upload xdg-desktop-portal(-gtk) from experimental to to unstable
  with pipewire enabled

- Versions of mutter and gnome-shell that make this practically useful
  are currently queued up in experimental for the GNOME 3.37.x transition
  (#969321) but their absence will not block migration

Thanks,
    smcv