[installation-guide] Wrong version displayed on buster page

Hi,

yesterday we got a message [1] on debian-l10n-german, that the installation-guide
for Buster [2] shows the wrong version (the one from Bullseye).

I have adapted the cron script which builds the installation-guide, to be able
to also build for oldstable as well [3] (only stable and testing were processed
there until now).

That helps to get new versions for oldstable built (if they should happen), 
and also gives the possibility to easily fix such issues as we have now
with the Buster one (a hiccup from the day when Bullseye was released).

As it seems, the build to correct this ran fine, so this issue should be 
solved soon...


FYI

Holger

[1] https://lists.debian.org/debian-l10n-german/2021/10/msg00031.html
[2] https://www.debian.org/releases/buster/amd64/
[3] https://salsa.debian.org/webmaster-team/cron/-/commit/b1bc80f627b9b3f9d20444beeed122c5ab9c8606