Replace deprecated exo with libxfce4ui 4.21.0
The removal of exo from the doc and build will take place at a later date, perhaps only at the end of cycle 4.22.
Related: exo!87 (merged)
The removal of exo from the doc and build will take place at a later date, perhaps only at the end of cycle 4.22.
Related: exo!87 (merged)