As a final step of upgrading a cluster, the new version must be set using the command:

SET CLUSTER SETTING version = (SELECT crdb_internal.node_executable_version())

This is an easy step to forget or skip unintentionally. We should display a warning, similar to our new version available warning, calling on the user to run this command.

This warning should only display if all the nodes are already updated to the new version.

Update (vilterp): we're going to upgrade the cluster version automatically (per @windchan7's RFC), but will display this banner if the user has opted out of the auto-upgrade.