Release severity: non-security patch should be `notice`

Created by: agh1

When the new system for getting version severity was implemented, it increased the default severity of patch releases to warning. Before, the default severity was notice.

This changes it back to notice.

In my mind @totten this is kind of urgent because users are now seeing "System Status: Warning" if they have 5.1.0 and thinking it's serious. Also, sites that have hushed the upgrade message at a severity of notice will suddenly have the upgrade message pop up because it's higher severity. If they hush it again, that will mean that actual warning-level messages are suppressed.