[FIX] account: add index for account.move.tax_cash_basis_move_id

Computing account.move.payment_state relies on this field. When
perfoming payment operations in bulk (like during an upgrade) the field
computation may become too slow.

On recent upgrade cases the abscense of this index caused delays ranging
from 5 hours to whole days in the upgrade of big DBs.

closes odoo/odoo#105794

Signed-off-by: Christophe Simonis <chs@odoo.com>