[GraphQL] Add merge_request detail to deprecated field property

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This issue was created from a suggestion by @cablett !27464 (comment 306848619) and !27464 (comment 306842282).

!27464 (merged) introduced a new deprecated property for GraphQL fields to deprecate the fields. The property is a Hash that currently contains: reason and milestone.

We should add merge_request to the deprecated Hash which would point to the merge request that introduced the deprecation. We can then update the auto-generated GraphQL schema docs to link to this merge request from the text "Deprecated in <milestone>".