[FIX] website: remove breaking z-index of parallax elements

Commit https://github.com/odoo/odoo/commit/c3c98e937e575853fe0b25fa6cb3c916e73fc591
introduced a z-index on parallax element to restrict the z-index -1 of
the s_parallax_bg in the snippets... this was however a bad idea as it
also restricts other positioned elements in there (like dropdown menus)
making them go under other elements in the page.

Fortunately, this z-index is in fact not needed at all since the
z-index -1 mentioned above has been removed with another commit:
https://github.com/odoo/odoo/commit/22745668cf66d802899dac526d810eba64b28c4b

However, the z-index -1 was necessary to force the s_parallax_bg under
static elements that go after it... and this was solved in 12.0 by
making those relative elements. See snippet refactoring in 12.0 with
https://github.com/odoo/odoo/commit/4dce6cc98b2b95a00249ac29245305e08f288e6c#diff-a50ffff96791f36c6dbe5c9273135e5aR783


This commit thus also contains a backport of that work.

closes odoo/odoo#39497

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

addons/website/static/src/less/website.snippets.less

@@ -418,7 +418,6 @@ div.carousel {
 // Parallax Theme
 .parallax {
     position: relative;
-    z-index: 0;
 
     .oe_structure {
         > :first-child {
@@ -429,6 +428,9 @@ div.carousel {
         }
     }
 
+    > * {
+        position: relative;
+    }
     > .s_parallax_bg {
         .o-position-absolute(0, 0, 0, 0);
         background-size: cover;