Update /v2/items schema to support the numeric secondary_suffix_item_id

[Archomeda]

The details.secondary_suffix_item_id property has traditionally always been a string, despite it being an integer.
With the latest schema change (2020-11-17T00:30:00.000Z), the custom string to int JSON converter can be phased out.

As a note, it also seems that version 5 of System.Text.Json also supports numeric values as strings, see dotnet/runtime#30255. So in case this pops up in a later stage, updating to version 5 is a better solution.

Reference: https://discordapp.com/channels/384735285197537290/384735523521953792/778065113948749834