[FIX] website: show mega menu over /my sidebar

Before this commit, the header elements such as mega menu and dropdown menu had
a z-index of 1000 (BS value for `.dropdown-menu`) while the portal sidebar has
a z-index of 1020 (BS value for `.sticky-top`), thus it was not hovering the
sidebar.

Note that in 14.0 the same fix was applied with 0f2cab4a.

Fixes #60666

closes odoo/odoo#61505

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>