[FIX] website: fix showcase snippet images stretched

Before this commit, if we replaced an icon of the showcase snippet
with an image, this image was streched depending on title size (if too
long and on 2 lines). Its because the align-self default value of a
flex item is strech.

task-2447087

closes odoo/odoo#67244

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>