mention service testing in the upgrade procedure (team#41388)

howto/upgrades/bookworm.md

@@ -172,6 +172,11 @@ See the "conflicts resolution" section below for how to handle
         printf "End of Step 8\a\n" &&
         shutdown -r +1 "bookworm upgrade step 8: testing reboots one final time"
 
+IMPORTANT: make sure you test the services at this point, or at least
+notify the admins responsible for the service so they do so. This will
+allow new problems that developed due to the upgrade to be found
+earlier.
+
 [this guide to free up space]: http://www.debian.org/releases/buster/amd64/release-notes/ch-upgrading.en.html#sufficient-space
 
 ## Conflicts resolution