qml: fix background is not aligned with software renderer in ArtistTopBanner

Since blur effect is not available with the non-RHI scene graph adaptation (software, and openvg adaptations), linear filtering was used here to approximate blur.

However with Qt 6, the texture does not seem to be aligned properly with the software adaptation. This is most likely a Qt bug, but since the effect is not really important for non-RHI adaptations, we can instead simply get rid of the effect and lower the opacity instead.

Request review @chub.