fix: use the correct design token color for the health chart background

This change swaps out the color of the health chart "unfilled" section
for `theme.palette.background.application`. This is the same color
that's used in the sketches, so it should apply better for dark mode.

Why? I noticed that the graph looks choppy in dark mode, so figured
I'd go and investigate. This update makes it look a lot smoother.

frontend/src/component/project/Project/ProjectStatus/ProjectHealth.tsx

@@ -118,7 +118,7 @@ export const ProjectHealth = () => {
                             cy='50'
                             r={ChartRadius}
                             fill='none'
-                            stroke={theme.palette.grey[300]}
+                            stroke={theme.palette.background.application}
                             strokeWidth={ChartStrokeWidth}
                             strokeDasharray={`${filledLength * circumference} ${gapLength * circumference}`}
                             strokeDashoffset={offset * circumference}