[FIX] website: increase the breakpoint of the 'Show on mobile' option

When using the 'Show on mobile' option, we can see that there is a
mismatch between the screen breakpoint at which the elements are
displayed like in mobile view (=> under 992px or `lg`) and the one that
is impacted by the 'Hide/Show' option (=> under 768px or `md`). This is
a problem because between these two breakpoints, the display is like in
mobile view but is not considered as such and so, hiding an element in
the mobile view (for example, if it does not look good in it) has no
effect until the screen reaches 768px.

This commit increases the screen breakpoint at which the 'Show on
mobile' option is applied, that is, up to 992px instead of 768px, in
order to be consistent with the display.

task-3110770

closes odoo/odoo#109053

Signed-off-by: Benoit Socias (bso) <bso@odoo.com>