In addition to the words that Marcus added to update-manager I've added
_("Sorry, there are no more upgrades for this system")
_("\nThere will not be any further Ubuntu releases "
"for this system's '%s' architecture.\n\n"
"Updates for Ubuntu %s will continue until "
"2023-04-26.\n\nIf you reinstall Ubuntu from "
"ubuntu.com/download, future upgrades will "
However, this dialog won't actually be displayed on systems running
20.04 so adding translations for it isn't a priority.