drop the ominous wording from the system update prompt

"the only step that changes software already on this machine" is true, and
reads like a warning about something dangerous rather than a description of
apt upgrade. The reasoning stays in the section comment, where it explains why
this is its own step; the prompt just says what it does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/setup/machine-setup/machine-setup.sh

@@ -90,7 +90,7 @@ if ! skip; then
   mapfile -t UPGRADABLE < <(pkg_upgradable)
 
   echo ""
-  info "System update — the only step that changes software already on this machine"
+  info "System update — upgrades installed packages to their latest versions"
 
   if ((${#UPGRADABLE[@]} == 0)); then
     echo "     to upgrade:   nothing, everything is current"