[FIX] website_sale: missing image on ecommerce product metadata

Before this commit:
No "image" metadata was available for products on the ecommerce /shop page.
As such search engines SEO will be less effective.
Using "Google Search Console" would give the message:
 `Missing field 'image'` as a "Top Warning"

Note:
This issue was introduced by:
https://github.com/odoo/odoo/pull/30656
and was partially solved by:
https://github.com/odoo/odoo/pull/37870/commits/c66892e65d2ae0ca31a686f65d4b517a9d7ffd0b

opw-2509546

closes odoo/odoo#74922

X-original-commit: af953e69de979794bebabdc4a88c7246f62b8843
Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>

addons/website_sale/views/templates.xml

@@ -191,7 +191,7 @@
                 <a t-att-href="product_href" class="d-block h-100" itemprop="url">
                     <t t-set="image_holder" t-value="product._get_image_holder()"/>
                     <span t-field="image_holder.image_1920"
-                        t-options="{'widget': 'image', 'preview_image': 'image_1024' if product_image_big else 'image_256'}"
+                        t-options="{'widget': 'image', 'preview_image': 'image_1024' if product_image_big else 'image_256', 'itemprop': 'image'}"
                         class="d-flex h-100 justify-content-center align-items-center"/>
                 </a>
             </div>