[REF] account: Vendor Bills: Refactor the lock date message

At the moment, the lock date message is generated in
_compute_tax_lock_date_message. We delegate it to a separate function
in order to use this message elsewhere as well.

Note: this refactor introduces minor changes in behaviour:
- we remove the second part of the lock date message (that mentions
  other lock dates than the most blocking one), because it is too much
  information.

- we calculate the new accounting date from the existing accounting date
  rather than from the existing invoice date. This should not change
  anything in practice, since the new accounting date will be the last
  day of the period after the lock date.

task-2823170

Part-of: odoo/odoo#121566