Upgrade GraphQL gem to 1.8.4 and replace echo function with a resolver

What does this MR do?

Upgrade GraphQL gem to 1.8.4 (stepping stone to 1.9.9)

The GraphQL::Function has been deprecated in favor of resolvers. Since we're upgrading the graphql gem and the echo function would need to get fixed, this is a good time to simply remove the use of functions altogether.

Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/62535

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers