[FIX] base_vat: wrong example vatnumber for CH

As described here:
https://www.estv.admin.ch/estv/fr/home/mehrwertsteuer/fachinformationen/steuerpflicht/unternehmens-identifikationsnummer--uid-.html



The "new" (since 2014) vat number has to be displayed as:
CHE 9 numeric digits plus TVA/MWST/IVA
e.g.: CHE-123.456.788 TVA

This commit removes the previous 6 digits vat number check and regex,
and provide accurate examples on the error message displayed if the
vatnumber given is wrong.

opw-2291581

closes odoo/odoo#59723

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>