There’s not much you can do but wait until the good folks at Packman update the GPG key.
That said, Packman may not be with us much longer, so I suggest you consider alternatives, such as VideoLAN repos or Flatpak.
Edit: Check the mailing list for more about updating the GPG key: https://lists.links2linux.de/pipermail/packman/2026-September/018344.html