[18.0][FIX] account_invoice_start_end_dates: Avoid same-label warning

[tobiaszehntner]

Odoo's account_accountant has two fields with the same label on the same model. Avoid warning by renaming the fields in Python while keeping the same label in the views.

Fields: account_accountant/models/account_move.py:446

Warnings:

WARNING odoo.addons.base.models.ir_model: Two fields (start_date, deferred_start_date) of account.move.line() have the same label: Start Date. [Modules: account_invoice_start_end_dates and account_accountant] 
WARNING odoo.addons.base.models.ir_model: Two fields (end_date, deferred_end_date) of account.move.line() have the same label: End Date. [Modules: account_invoice_start_end_dates and account_accountant] 

[OCA-git-bot]

Hi @alexis-via,
some modules you are maintaining are being modified, check this out!