Currently, an `incorrect warning` message is shown when sending an invoice to 
`DIAN` or `Carvajal`, if the invoice date is 6 days before today, even though this 
date should be considered valid.

**Steps to reproduce:**
- Install the `l10n_co_dian` module and switch to the `CO company`.
- Go to `Invoicing` and create a new invoice with `taxes`.
- Set the `Invoice Date` to 6 days before today.
- Click `Confirm` > `Send`, ensure `DIAN` is selected, and `send` the invoice.
- Observe the warning message.

**Observation:**
`The issue date can not be older than 5 days or more than 5 days in the future.`

**Root cause:**
At [1] and [2], the allowed invoice date range is incorrectly computed, using 
`5 days in the past` and `10 days in the future`. This does not match the Colombian 
regulations and triggers incorrect validation errors for valid invoice dates.

These checks are intentionally implemented in both modules because they apply
at different stages and for different providers:
1) `l10n_co_dian`
When `DIAN: Free service` is selected as the `Electronic Invoicing Provider`
from the `Invoicing Settings`, the date constraint is evaluated at `send time`.
After the invoice is created, the validation is performed when the user sends 
the invoice to `DIAN`, and a blocking error is raised as a `UserError` if the 
invoice date is outside the allowed range.

2) `l10n_co_edi`
When `Carvajal` is selected as the `Electronic Invoicing Provider`, the same 
rule is checked `at the confirmation time` of the draft invoice. In this case, 
the validation results in a `chatter message`, not a blocking send-time error.

**Fix:**
This commit updates the date constraint logic to allow invoices dated up to 
`6 days before and 6 days after` the current date, in accordance with the 
DIAN specification described in Anexo Técnico – Documento Soporte No Obligados, 
page 59, at [3].

<img width="1089" height="120" alt="DIAN" src="https://github.com/user-attachments/assets/408c5ccc-5fed-4585-a81e-dce4ccb98b40" />

[1]:
https://github.com/odoo/enterprise/blob/2217827c0989fc5fc8d52a6a9b6f6c6d00b9773f/l10n_co_dian/models/account_edi_xml_ubl_dian.py#L672-L679

[2]:
https://github.com/odoo/enterprise/blob/913e55abc4a9aa58509aa2a60d378fb552de554d/l10n_co_edi/models/account_edi_format.py#L574-L603

[3]:
https://www.dian.gov.co/impuestos/factura-electronica/Documents/Anexo-Tecnico-Documento-Soporte-No-Obligados.pdf

opw-5482555

Forward-Port-Of: odoo/enterprise#107255
Forward-Port-Of: odoo/enterprise#105763