[FIX] account: Change default readonly of invoice_user_id to False

A previous commit(ea8acc17b4)
added a compute on invoice_user_id fields to
guess the Sale Person. The problem is that by adding the compute
on the field, the readonly default value changes for True.
It causes that user can't change the sale person anymore.

no task id

closes odoo/odoo#139035

X-original-commit: af5e3eedf90d64462afc625b30dec742361389f5
Signed-off-by: John Laterre (jol) <jol@odoo.com>
Signed-off-by: Florian Gilbert (flg) <flg@odoo.com>

addons/account/models/account_move.py

@@ -510,6 +510,7 @@ class AccountMove(models.Model):
         tracking=True,
         compute='_compute_invoice_default_sale_person',
         store=True,
+        readonly=False,
     )
     # Technical field used to fit the generic behavior in mail templates.
     user_id = fields.Many2one(string='User', related='invoice_user_id')