[FIX] web: grouped kanban header shouldn't overlay dropdowns

Before this commit, in grouped kanban, when the "switch view" dropdown
(on smaller screens) is open, it is hidden behind the kanban group's
header.

This commit fixes it by setting the kanban group header's `z-index` to
only overlay its siblings (cf. this groups' records) but not the
ControlPanel's dropdowns. The static value "1" allows to elevate locally
the header above the records instead of using the Bootstrap's
`$zindex-sticky` variable which is aimed at raising an element (like a
navbar) above any other content.

X-original-commit: 1799e02cd56a26e48665043ca23ac1c735996c50
Part-of: odoo/odoo#100759

addons/web/static/src/views/kanban/kanban_controller.scss

@@ -559,7 +559,7 @@
 
             .o_kanban_header {
                 position: sticky; // keep it visible
-                z-index: $zindex-sticky;
+                z-index: 1;
                 background-color: inherit;
             }
         }