[UI] Groups Stream Tab - Expandable tab menu items are being overlapped

Problem/Motivation

On the Group Stream Tab, clicking on the arrow to view hidden tabs seems to display the tabs behind the post section.

Steps to reproduce

1. Login and navigate to a group
2. Click on the arrow to display hidden tab menu items
3. See that the expanded menu is hidden behind the posts section

This issue is caused by https://www.drupal.org/project/socialbase/issues/3274735 → -> As here we are increasing `z-index` for Group stream element.

Proposed resolution

Increase `z-index` on secondary navigation menu so that it's stacked up correctly with child elements.

Remaining tasks

N/A

User interface changes

Expandable tab menu items on Group Stream page is fixed to show up correctly with stacked elements.

API changes

N/A

Data model changes

N/A