QWaylandAdwaitaDecoration: fix border color to better match Adwaita

Update the color of window borders to better match current Adwaita style
and avoid currently non-visible border causing dialogs to blend with
the background window.

Backported from QAdwaitaDecorations on behalf of Ethan Chappel.

Fixes: QTBUG-146200
Pick-to: 6.12 6.11
Change-Id: Iaaaa3acbeae79637aa882a10a71c5fbbc063ef12
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>