Acho que você pode editar /etc/update-manager/release-upgrades
> # default behavior for the release upgrader
> #
>
> [DEFAULT]
> # default prompting behavior, valid options:
> # never - never prompt for a new distribution version
> # normal - prompt if a new version of the distribution is available
> # lts - prompt only if a LTS version of the distribution is available Prompt=normal
> Prompt=never
Você provavelmente deseja configurá-lo para nunca.