[4.0] Dropdown user menu (#22610)

PR for #22561

A recent PR was merged to ensure that the postinstallation messages dropdown is not hidden behind the toolbar. However it wasn't quite correct and the user menu was still hidden

With this PR (not testable with patchtester) both the user menu and the postintallation messages haave the correct z-index

administrator/templates/atum/scss/blocks/_header.scss

@@ -53,7 +53,7 @@
     }
   }
 
-  .dropdown-notifications {
+  .dropdown-menu {
     min-width: 280px;
     padding: 0;
     border: 0;