After the change to "Capacity degradation" was announced on
https://pointieststick.com/2019/08/24/kde-usability-productivity-week-85/, it turned out
that nobody could understand the new string either. :(
This patch changes it to something much clearer: suggested by a user: "Battery health".
This string has the advantage that it does not require knowing that batteries degrade,
and uses the same percentage scale as other operating systems.