[FIX] mrp: set backorder name with custom sequence name

If the prefix of the production sequence contains some dashes, it can
lead to undesirable behaviors

To reproduce the error:
(Enable debug mode)
1. Settings > Technical > Sequences & Identifiers > Sequences, edit "San
Francisco Sequence production":
    - Prefix: "WH-MO-"
2. Create a MO
    - Quantity: 2
3. Confirm, Edit:
    - Quantity: 1/2
4. (Keep MO's name in mind and) Validate + Create backorder

Error: Backorder's name is "WH-MO-002" instead of "WH-MO-0000X-002".
Going back to the initial MO, its name became "WH-MO-001" (was
"WH-MO-0000X")

OPW-2618971

closes odoo/odoo#75130

Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>