[FIX] base: missing MIME-Version header

Send an rich HTML email from Odoo, it lands in spam whereas it would
land in inbox in 13.0.

The problem is due to a missing "MIME-Version: 1.0" header on the email.
This header is correctly set on both the html and text alternatives of
the messages but it should be set on the enveloppe too.

The problem is present in the newer EmailMessage mail API of python that
is used since 14.0. Using the newer API, it doesn't set the header on
the enveloppe itself.

This reverts commit 8663f1e2.

opw-3098621

closes odoo/odoo#109056

X-original-commit: 04e91654
Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>
Signed-off-by: Julien Castiaux <juc@odoo.com>