feat: upgrade myinfo-gov-client to 4.0.0

[mantariksh]

Upgrades @opengovsg/myinfo-gov-client to the latest version, and makes the necessary update to accommodate the breaking changes.

The latest version includes a breaking change which makes nested scope data optional (see PR). This is because there is no way of knowing whether a given MyInfoPerson data object contains the given nested scopes without knowing which scopes were requested in advance.

In FormSG's case, vehicles.vehicleno is the only nested scope we deal with, and this property is only accessed if the vehicleno field is requested, so the property access is safe. Hence the additional ? optional chain was added purely to accommodate the new types from myinfo-gov-client.

Manual tests

• Add a Vehicle Number field to a MyInfo form. Test that the form can be submitted for a MyInfo profile with no prefilled vehicle number.
• Use a profile with a prefilled vehicle number and check that the form can be submitted without editing the field.
• Use a profile with a prefilled vehicle number and edit the field. Check that the form can still be submitted.

[seaerchin]

lgtm

[karrui]

lgtm

[chowyiyin]

lgtm