I know I've seen this documented somewhere before, but what's the option to keep update-manager from prompting for updates except for LTS-to-LTS releases?
If you go to Ubuntu > System > Administration > Update Manager, you'll see a Settings... button at the bottom left of the dialog. In the dialog, switch to the Updates tab and uncheck the updates checkboxes and set Release Upgrade to "Long Term Support Releases Only".
It should look something like:
See the community docs for more information.
Posting as an answer what @flickerfly said in a comment: the way to do it without a GUI is to change the value of
The possible values are (copied from the file itself):
never: Never check for a new release.
normal: Check to see if a new release is available. If more than one new release is found, the release upgrader will attempt to upgrade to the release that immediately succeeds the currently-running release.
lts: Check to see if a new LTS release is available. The upgrader will attempt to upgrade to the first LTS release available after the currently-running one. Note that this option should not be used if the currently-running release is not itself an LTS release, since in that case the upgrader won't be able to determine if a newer release is available.