x11-wm/xfce4: Switch default icon set to x11-themes/xfce-icons-elementary

After the update x11-themes/adwaita-icon-theme update to version
42.0 (commit 14ac2cbef6484e85669c99a5a8388ea4627f029f) XFCE is not
compatible with it anymore, due to icons being renamed/removed from
the theme.

To work around this, change the default icon theme installed with
XFCE to x11-themes/xfce-icons-elementary.

Users with XFCE desktops already configured to use adwaita-icon-theme
need to install another theme and switch to it manually using the
XFCE settings "appearance" application, see UPDATING entry 20230924.

PR: 274057