fix: new header z-index style (#9541)

Author: Tymek

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

@@ -97,7 +97,6 @@ const DraftBannerContent: FC<{
 };
 
 const StickyBanner = styled(Sticky)(({ theme }) => ({
-    borderTop: `1px solid ${theme.palette.warning.border}`,
     borderBottom: `1px solid ${theme.palette.warning.border}`,
     color: theme.palette.warning.contrastText,
     backgroundColor: theme.palette.warning.light,

frontend/src/component/menu/Header/Header.tsx

@@ -34,7 +34,6 @@ const HeaderComponent = styled(AppBar)(({ theme }) => ({
     padding: theme.spacing(1),
     boxShadow: 'none',
     position: 'relative',
-    zIndex: 300,
     paddingRight: theme.spacing(9),
     [theme.breakpoints.down('lg')]: {
         paddingLeft: theme.spacing(1),