Decrease waiting time in mintupdate-automation-autoremove.timer

[Scumpeter]

Is your feature request related to a problem? Please describe.
The function "Remove obsolete kernels and dependencies" has no effect for users who do not regularly use their computer for over 3 hours. This does not only clutter the /boot partition but also confuse users (see #648).

Describe the solution you'd like
The timer should be set to a shorter time after startup. My suggestion is 30 minutes or shorter. At most 60 minutes.

Describe alternatives you've considered
Alternatively mention the delay in the description of the setting.

Thank you very much for your effort.