Fix z-index issue

This commit lowers the z-index of the .merge-request-tabs-holder class
to avoid rendering conflicts with the mini pipeline status dropdown
on the merge request page.

app/assets/stylesheets/pages/merge_requests.scss

@@ -806,7 +806,7 @@
 
 .merge-request-tabs-holder {
   top: $header-height;
-  z-index: 300;
+  z-index: 250;
   background-color: $white-light;
   border-bottom: 1px solid $border-color;
 

changelogs/unreleased/58999-z-index-issue-on-pipeline-dropdown.yml

+---
+title: Fix issue that caused the "Show all activity" button to appear on top of the
+  mini pipeline status dropdown on the merge request page
+merge_request: 26274
+author:
+type: fixed