[FIX] auth_signup: fix notification close button position

Since the bootstrap5 merge, dismissible alerts need to have a special
class for its close button to be positioned correctly. That button also
no longer needs to contain its own cross as it is added by bootstrap.
The class for such close buttons has also changed. This commit fixes
that.

closes odoo/odoo#97939

Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>

addons/auth_signup/views/res_users_views.xml

@@ -18,8 +18,8 @@
                 </xpath>
 
                 <xpath expr="//sheet" position="before">
-                    <div class="alert alert-success text-center o_form_header" attrs="{'invisible': [('signup_valid', '!=', True)]}" role="status">
-                        <a class="close" data-bs-dismiss="alert" href="#" aria-label="Close"><i title="Close" class="small fa fa-times"/></a>
+                    <div class="alert alert-success text-center o_form_header alert-dismissible" attrs="{'invisible': [('signup_valid', '!=', True)]}" role="status">
+                        <button class="btn-close" data-bs-dismiss="alert" aria-label="Close"/>
                         <div attrs="{'invisible': [('state', '!=', 'active')]}">
                             <strong>A password reset has been requested for this user. An email containing the following link has been sent:</strong>
                         </div>