fix: draft banner zIndex (#5124)

Tiny zIndex fix for the draft banner for a regression introduced in
#5088

### Before - Draft banner is displayed on top of the profile popup:

![image](https://github.com/Unleash/unleash/assets/14320932/63865f01-9bbe-42f1-9cc4-85c3c985334c)

### After - Profile popup displays on top of the draft banner:

![image](https://github.com/Unleash/unleash/assets/14320932/565e1017-5163-445d-bc0c-ee957023241b)

frontend/src/component/layout/MainLayout/DraftBanner/DraftBanner.tsx

@@ -104,6 +104,7 @@ const StickyBanner = styled(Sticky)(({ theme }) => ({
     borderBottom: `1px solid ${theme.palette.warning.border}`,
     color: theme.palette.warning.contrastText,
     backgroundColor: theme.palette.warning.light,
+    zIndex: 250,
 }));
 
 export const DraftBanner: VFC<IDraftBannerProps> = ({ project }) => {