[FIX] website: fix animations in popups without backdrop

Since the scrollbar was moved from the window to .modal-content at [1],
animations inside popups without backdrops would not be working anymore
because the scroll was not checked against the right element.

This commit fixes this by using the right scroll element's scrollTop.

task-3328419

[1] https://github.com/odoo/odoo/commit/cb7cf77



closes odoo/odoo#122622

Signed-off-by: Outagant Mehdi (mou) <mou@odoo.com>