At noted by Marek [1] GPG keyrings for verifying 4.1+ packages only
started being available in 4.1 templates. Telling people to fetch
the keyrings through another method would make instructions too
complicated. For this reason it seems simpler to just tell people to
use the new templates instead.
[1]: https://github.com/QubesOS/qubes-doc/pull/1345#issuecomment-1803517947
Tell users to upgrade the software sources. Otherwise they may miss
on critical integrations for 4.2 like VMExec (for the updater to work)
and most importantly, after 4.1 is EOL, it'll silently stop resolving
qubes package upgrades even though the template is not yet EOL.
