Steps to follow to reproduce the bug:
- Install Inventory and Purchase
- In the inventory settings, Enable the "Landed Costs" option
- Go to Purchase > create a Service type product
- In the Purchase tab > Enable "is landed cost" option and choose "by quantity" in the default split method
- Go to Orders > Requests for quotation > Create a new RFQ > add Any product and confirm the order
- Validate receipt of the product
- Create a bill > add the Service you created at the beginning
- Create Landed Costs

Problem:
we do not use the split method landed cost of the product, we automatically set it to "equal"

opw-2465753

closes odoo/odoo#69973

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>

For customer invoices:
* do not set default date because if you prepare an invoice (and it gets
  a default date), then validate it the next day, the date will be wrong
* set the date when posting if it wasn't set, because why not?

For vendor bills:
* do not set default date because you rarely encode a bill at the bill
  date. Forcing the user to enter it reduces risks of user error
  (duplicated vendor bill)
* do not set it when posting, same reason.

opw-2492862
Related #68368
Closes #68367

closes odoo/odoo#68689

Related: odoo/enterprise#17502
Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>