[FW][IMP] mail: allow to remove plus addressing from bounce return addresses

Since odoo/odoo@f4524f03c32a27b1899562a71cad6f491bfe44ce plus addressing is not used anymore
for handling bounces. Indeed it relies on references / in reply to to find
original message that bounced. It is therefore not necessary to enforce the
use of plus addressing.

As some provider do not support plus addressing as a way to contact left-part
of email with sub-informations people should have a way to deactivate plus
addressing used in bounce aliases.

To preserve backwards compatibility for stable versions old behavior is
retained unless a new `mail.bounce.alias.static` ICP is set with a truthy
value.

Fix https://github.com/odoo/odoo/issues/71242 by dropping requirement of plus addressing.

@Tecnativa TT29827
Closes #71242
Task ID-2547347
PR odoo/odoo#72347

X-original-commit: df2d955b