[Battery Monitor] Call battery capacity "Capacity degradation" for better clarity
Companion patch to D23270
The same logic explained in D23270 applies here: using the text "Capacity degradation"
is much clearer than "capacity" when referring to how much of the battery's original
capacity is remaining due to degradation.
Subscribers: aspotashev, filipf, plasma-devel
Differential Revision: https://phabricator.kde.org/D23271