Update z-index and height properties in useMenuHeaderStyle hook

- Changed z-index from a static value to a calculated value based on the EUI theme.
- Updated height to min-height to ensure proper layout handling.

These adjustments enhance the responsiveness and styling of the menu header.

Author: paulinashakirova

src/platform/kbn-ui/side-navigation/src/hooks/use_menu_header_style.ts

@@ -24,10 +24,10 @@ export function useMenuHeaderStyle() {
 
     position: sticky;
     top: 0;
-    z-index: 1;
+    z-index: calc(${euiTheme.levels.content} + 1);
     padding: ${euiTheme.size.base} var(--horizontal-padding) ${euiTheme.size.xs}
       var(--horizontal-padding);
     margin: 0 1px;
-    height: var(--secondary-menu-header-height);
+    min-height: var(--secondary-menu-header-height);
   `;
 }