fix(nut-exporter): Fix some more power ranges for UPS dashboard

I'm currently learning a lot about UPSes and the european power grid.

Example: Linepower is: 230 V ± 23 V with a frequency of 50 Hz ± 0,2 Hz

I didn't know the range was that big.

And of UPS batteries rated for 12 voltes, it's comepletely fine to reach
14 volts when fully charged.