[IMP] account: introduce new grouping threshod field on reports

Since these commits:
- https://github.com/odoo/enterprise/commit/d6e771f1f9ab4440e47188ea24c2fc80f58c2919
- https://github.com/odoo/enterprise/commit/51bc88bdec323035c5d1949a8ab76e2c032b26f2

Several reports were using a config parameter to define
a threshold used to group report lines.

The goal here is to replace this parameter by a new
`prefix_groups_threshold` field on `account.report`.

task-3330099

closes odoo/odoo#122138

Related: odoo/upgrade#4694
Related: odoo/enterprise#41362
Signed-off-by: Olivier Colson (oco) <oco@odoo.com>

addons/account/models/account_report.py

@@ -46,6 +46,7 @@ class AccountReport(models.Model):
     )
     load_more_limit = fields.Integer(string="Load More Limit")
     search_bar = fields.Boolean(string="Search Bar")
+    prefix_groups_threshold = fields.Integer(string="Prefix Groups Threshold")
 
     default_opening_date_filter = fields.Selection(
         string="Default Opening",