[FIX] mail: schedule an activity button not visible in mobile

Before this commit, text color was in grey because of a css
inherited rule applied by Bootstrap when there is no href attribute.
In mobile, text color was in white as the background and it was not
possible to see the text.

Now, we transform this link into button to restore the previous
behavior and keep it visible in mobile.

closes odoo/odoo#28061

addons/mail/static/src/xml/web_kanban_activity.xml

@@ -78,9 +78,9 @@
     </div>
     <div class="dropdown-divider mt-0"/>
     <div role="menuitem" class="o_schedule_activity dropdown-header py-1 text-center">
-        <a role="button" class="btn btn-secondary mb-1">
+        <button class="btn btn-secondary mb-1">
             <i class="fa fa-plus fa-fw"></i><strong>Schedule an activity</strong>
-        </a>
+        </button>
     </div>
 </t>