[IMP] base: ir.mail_server order by sequence

The ir.mail_server Model includes a sequence field, promising that the
lower sequence number have higher priority when multiple server
configurations are available.

The servers are already used based on that parameter since 00bd7274c4 but this
was not enforced at the _order level.
Make it coherent everywhere

closes odoo/odoo#28313

odoo/addons/base/models/ir_mail_server.py

@@ -129,6 +129,7 @@ class IrMailServer(models.Model):
     """Represents an SMTP server, able to send outgoing emails, with SSL and TLS capabilities."""
     _name = "ir.mail_server"
     _description = 'Mail Server'
+    _order = 'sequence'
 
     NO_VALID_RECIPIENT = ("At least one valid recipient address should be "
                           "specified for outgoing emails (To/Cc/Bcc)")