community/xdg-desktop-portal: downgrade to 1.18.4

1.19.0, and all 1.19.x releases, are marked as unstable upstream. (Looks like they're following an "even version is stable, odd version is unstable" release schedule). Downgrade it back to a stable version.

Newer xdp seems to be causing some issues, see https://gitlab.postmarketos.org/postmarketOS/pmaports/-/issues/3501. fixed by even newer xdp, but this seems more correct...

This reverts commit c027f54b.