[UX] Set the default max_duration value to off #2349

Problem:
Currently, dev environments and tasks have different defaults for max_duration. Most users don't know these defaults and as a result, their runs inconsistently fail and it's difficult to understand why and how to avoid it. All is causing strong frustration.

Solution:
Just like previously we did for the spot policy, let's set the default max_duration value to off. Them, if users want to control it, they will do it explicitly.