Use upstream's Xsession
Closes: https://bugs.debian.org/1120328
Ubuntu does have one additional Xsession patch, but it adds translatable strings so I hesitated to apply it. Perhaps it's worth it anyway?
Testing
The only testing I did was
sudo apt install xfce4 lightdm-
And then I rebooted and was able to log into the default Xfce session. There is a new Xfce Wayland session that didn't work for me but I didn't necessarily expect it to.
This means I didn't test any of the custom configuration in /etc/X11/Xresources/ or anywhere else. I also didn't test the fallback when an Xsession fails to load.