[Battery Monitor] Call battery capacity "Capacity degradation" for better clarity

Summary:
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.

Test Plan:

Reviewers: VDG, broulik, Plasma, filipf

Reviewed By: VDG, broulik, Plasma, filipf

Subscribers: aspotashev, filipf, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23271