[MERGE][FIX] product: make copy of variants just copy template

Variants are generated depending on the configuration of attributes
and values on the template, so copying them does not make sense.

For convenience the template is copied instead and its first variant is
returned.

closes #38151

closes odoo/odoo#38303

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>