Cleanup workspaces graphql specs/resolvers

What does this MR do and why?

Several cleanups related to Workspaces graphql resolvers:

1. Add missing Group.workspacesClusterAgents field, deprecate Group.remoteDevelopmentClusterAgents field
2. Update README under ee/spec/requests/api/graphql/remote_development to provide more info and guidance
3. Add missing specs under ee/spec/requests/api/graphql/remote_development/cluster_agent/workspaces_agent_config
4. Refactor existing specs under ee/spec/requests/api/graphql/remote_development to match existing patterns
5. Change structure of namespaces under ee/app/graphql/resolvers/remote_development to more closely match structure of corresponding request specs.
6. Update and add missing entries to tests.yml and scripts/verify-tff-mapping
7. Change ee/app/finders/remote_development/cluster_agents_finder.rb to return empty array rather than raise error in the case of unauthorized access (to match existing graphql standard patterns).
8. Other various cleanups

References

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• Add agent config type, resolve, finder with per... (!158587 - merged)
• https://gitlab.com/gitlab-org/workspaces/gitlab-workspaces-docs/-/blob/45fdeec0392c6b1d85a9b71d613bd3df2b1de0da/doc/tech-designs/2024-01-23-support-group-agent-authorization.md

Followup issue: Remove clusterAgent.remoteDevelopmentAgentConfi... (#509049 - closed)

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