[GraphQL] Discussions: use Global-ID scalar

What does this MR do?

Uses GlobalID scalar type for Discussions GraphQL code.

Contributes to #259658 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content