[FIX] account: bank statement with many AML

- Create a bank statement with more than 80 lines (therefore more than
  80 AML)
- Reconcile the lines
- Go back to the bank statement, and try to add a new line

An error arise:
"You cannot do this modification on a posted journal entry, you can just
change some non legal fields"

The error arise on the `move_line_ids` field on which the web client
tries to write.

Actually, this field is hidden since it is only used in the domain of a
stat button. We can make it read-only.

opw-1921138

closes odoo/odoo#30266