Steps to reproduce

1. Make a LineAnnotation with some text.
2. Set StrokeThickness to a non-zero value and Stroke to an OxyColor.
3. Observe that you can't set a Background like with TextAnnotation
4. At runtime, observe that the Stoke and StrokeThickness properties are ignored.

Platform: Windows 11
.NET version: 6.0.6
OxyPlot version: 2.1.0+ custom-built from e3c42f7

Expected behaviour

I can set a Background property on my LineAnnotation and the Stoke and StrokeThickness properties aren't ignored.

Actual behaviour

I can't set a Background, and the stroke ones are ignored at runtime (no border box is drawn)