Replace libsoup with libsoup3
This matches the change to webkitgtk-6.0. Related upstream commits: https://github.com/FontManager/font-manager/commit/5cbec5eab33f6e64c27ead07cdee0098431ad6b9 https://github.com/FontManager/font-manager/commit/6d941d00dae897e1dc90d24e2897f985e68ece86