update desktop environment support docs

[khewweifeng]

https://github.com/system76/docs/blob/70386a946d243e974062ceeb9ad1ac63cbee9254/content/desktop-environment.md#:~:text=sudo%20apt%20autoremove%20%2D%2Dpurge%20kde%2Dstandard

sudo apt autoremove --purge cinnamon-desktop-environment did not remove completely (it still can be selected from login screen desktop environment switcher

need to run additional command sudo apt autoremove cinnamon to remove cinnamon-desktop environment completely