[FIX] mail: use correct template attribute

Custom CSS should be in style attribute

Closes odoo/odoo#37638

closes odoo/odoo#37666

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

addons/mail/data/mail_data.xml

@@ -86,7 +86,7 @@
         <li><t t-esc="tracking[0]"/>: <t t-esc="tracking[1]"/> -&gt; <t t-esc="tracking[2]"/></li>
     </t>
 </ul>
-<div t-if="signature" t-raw="signature" class="font-size: 13px;"/>
+<div t-if="signature" t-raw="signature" style="font-size: 13px;"/>
 <p style="color: #555555; margin-top:32px;">
     Sent
     <span t-if="company.name">