Add approvedAt to GraphQL merge request interaction

What does this MR do and why?

The REST API exposes approved_at for merge request approvals (API::Entities::Approvals, aliasing Approval#created_at), but GraphQL has no equivalent. This adds an approvedAt field to UserMergeRequestInteractionType, mirroring the existing approved boolean field.

Relates to #618872 (closed)

Changes

• Users::MergeRequestInteraction#approved_at returns the matching Approval record's created_at; approved? refactored to reuse the same lookup instead of scanning approvals twice.
• Types::UserMergeRequestInteractionType exposes approvedAt (Types::TimeType, nullable).
• Updated model and GraphQL type specs.
• Updated doc/api/graphql/reference/_index.md.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.