Tell users of the terminal-based updater about the correct way to update to recent versions in the startup update check

The terminal-based updater in versions 25.07 to 25.09 can no longer be used to update to recent versions due to packaging changes. We have a pinned forum announcement about that, but an in-app message would be more useful.

AnkiWeb supports sending one-time messages to users via the update check run on startup. We can use that to notify affected users.