Deprecate dedicated requirement object (legacy) IID in GraphQL API

See &9203 (comment 1170193578) for a breakdown.

After we have support of both dedicated requirement IID and work item IID to refer to a requirement, we can begin deprecating dedicated requirement object references (e.g. IIDs) and supporting customers that use them to move across to use work item IIDs.

I think once we've removed the legacy Requirement IID, we can then proceed with using the Work Item GraphQL query instead of the Requirements filter, and then Deprecate the Requirements API (#345308). It also means we maybe don't have to add legacy IID support to Work Items API.

GraphQL filtering will involve using deprecated: parameter to the filtration argument. The list itself could possibly put a DEPRECATED sort of warning next to the old IID? See #382141 (closed)