Migrate to libsoup3

I stumbled over this topic because of a mail on the Fedora Development mailing list: https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/L7TNU4MSIAMNM2N2OGGJ3I46DEA5MKV7/

Because of this I realized that the xfce4-weather-plugin depends on libsoup currently.

Since there was no new release to libsoup since two years now [1], I suggest to give an eye on it to figure what needs to be done to migrate it to libsoup3.

[1] https://gitlab.gnome.org/GNOME/libsoup/-/tags?search=%5E2.*&sort=updated_desc