Use the same code for `AvailabilityCode` and `ProductAvailability` in ONIX

[ja573]

We currently harcode ProductAvailability as 99, but that's causing some confusion

In ONIX 3.0 and later, the data element is not present, and the lifecycle is captured in two separate data elements – and 3 4. The former is the ‘publisher’s eye view’ of the lifecycle, and the latter is the lifecycle as seen from the supplier’s viewpoint (the perspective of the publisher’s distributor or of a wholesaler).