Updates the P45 Values on the Opening Balances for an Employee.
This would ideally be done when the Opening Balances are updated but if payruns have already been started for the employee then Opening Balances can't be updated - hence this API call just to update the P45 values.
There must be a currently open PayRun for the employee.
Only the PreviousEmployerGross and PreviousEmployerTax properties of the submitted OpeningBalances model are updated.