[FIX] purchase: correct amounts for purchase report multi-company/currency

Selection of multiple companies to view multi-company data was added
in v13, but at the time there was no way to have reports correctly
take into account currency rates when also working with multi-currency.
v14 onwards is able to correctly apply the currency rates, therefore we
fix the purchase report to do so.

Steps to reproduce:
1. Start with existing demo data + add a new company with currency = EUR
2. Activate multi-currencies + set a currency rate (not 1) for Euro to $
3. Open Purchase Report (Purchase > Reporting > Dashboard)
4. Activate demo company + new EUR company
5. Switch between USD and EUR company as selected company

Expected result:
  Dashboard monetary quantities switch between $ and EUR, i.e. both the
  amount changes according to current exchange rate and symbol.

Actual result:
  Currency symbol changes, but amount stays the same.

closes odoo/odoo#75651

Signed-off-by: Arnold Moyaux <arm@odoo.com>