Update Debian template intro to reflect that Xfce is the default

[andrewdavidwong]

Now that the Xfce templates are the default (QubesOS/qubes-issues#7784 (comment)), this part is no longer accurate.

P.S. -- The Debian version is out of date (13 is now the latest version), but I don't know how to change the first variablized mention (|debian-version| (“|debian-codename|”)), so I didn't change the subsequent mentions (template names) to avoid inconsistency.

[parulin]

P.S. -- The Debian version is out of date (13 is now the latest version), but I don't know how to change the first variablized mention (|debian-version| (“|debian-codename|”)), so I didn't change the subsequent mentions (template names) to avoid inconsistency.

The change should be made in conf.py. I introduced this in the first place but I am now reconsidering the whole thing:

1. remove all those substitutions because that's not contributor-friendly
2. create and document something like a suffix.rst document that would be read by conf.py and provide the contributors an easier way to edit those substitutions