Fix scoped board milestone/iteration Timebox global ID

What does this MR do and why?

Fixes how Timebox structs are represented as global IDs, they should retain the class name that includes Timebox. For milestones for example, this will change:

gid://gitlab/Timebox::TimeboxStruct/-1 => gid://gitlab/Milestone/-1

The MR also fixes this for boards scoped to an iteration.

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Related to #344779 (closed)