🍒 LabelBackground: improve text readability

Previously it would alternate between the text color and highlighted text color, but this didn't work since the highlighted text color is only guaranteed to be readable on top of something using the highlight color, and that couldn't be guaranteed here.

By contrast, the text color always works here, especially with a small tweak to the colored area to desaturate it a tiny bit.

BUG: 492656 FIXED-IN: 6.2.4

(cherry picked from commit 2ea20cae)

Co-authored-by: Nate Graham nate@kde.org