fix: message banner zIndex (#5015)

After aligning with @nicolaesocaciu, this fixes the message banner
zIndex.

Co-authored-by: Nicolae <nicolae@getunleash.ai>

frontend/src/component/messageBanners/MessageBanner/MessageBanner.tsx

@@ -24,7 +24,7 @@ const StyledBar = styled('aside', {
         fontSize: theme.fontSizes.smallBody,
         ...(sticky && {
             top: 0,
-            zIndex: theme.zIndex.sticky,
+            zIndex: theme.zIndex.sticky - 100,
         }),
     }),
 );