Remove old Orbit GraphQL fields and update Orbit API docs

What does this MR do and why?

Removes the old GraphQL fields and mutations that Query.orbit replaced, which !261894 (merged) announced. This MR covers only those removals, not any other part of that follow-up. It also lists each Orbit REST endpoint's status in the API docs (see !264917 (merged)) and corrects the offering line.

References

• Follows !261894 (merged) and !264917 (merged)
• Endpoint plan: gitlab-org/orbit/knowledge-graph#263

How to validate

1. Run bundle exec rspec ee/spec/requests/api/graphql/mutations/analytics ee/spec/requests/api/graphql/analytics/orbit ee/spec/graphql/ee/types ee/spec/requests/api/graphql/group ee/spec/requests/api/graphql/groups_query_spec.rb.
2. Confirm doc/api/graphql/reference has no knowledgeGraphEnabled, orbitAccess or orbitTrial.

Agent context

• The excluded-group mutations and resolver moved because Query.orbit and the orbit* mutations reused the removed classes, so deleting them outright would break those.
• Request specs for orbitTrial and orbitAccess were ported to Group.orbit.trial and Group.orbit.access, and the excluded-groups query spec was kept on Query.orbit. public/-/graphql/introspection_result.json is regenerated.
• Removed fields: Group.knowledgeGraphEnabled, knowledgeGraphAvailable, orbitAccess, orbitTrial, Query.knowledgeGraphExcludedNamespaces, and the mutations knowledgeGraphExcludedNamespaceCreate / Destroy. All were experiment fields, so no deprecation period applies.
• No frontend callers: nothing under app/assets or ee/app/assets queries them. The knowledgeGraphEnabled key in graph_transform.js is a local object property fed from orbit.enabled.
• The orbitExcludedGroupCreate / Destroy mutations and orbit.excludedGroups subclassed or reused the removed classes, so the code moved into Mutations::Analytics::Orbit::ExcludedGroup* and Resolvers::Analytics::Orbit::ExcludedGroupsResolver. Their specs lost the dual old/new loops.
• Specs for the removed fields were deleted. The remaining ones that selected knowledgeGraphEnabled use orbit { enabled }.
• Docs: new "Endpoint status" table in doc/api/orbit.md matching the lifecycle settings in ee/lib/api/orbit/data.rb, Experiment status on the grep section, and offering now lists Self-Managed and Dedicated. The two removed mutations are dropped from the fine-grained token doc. The Duo context page linked the deleted page, so it now links https://docs.gitlab.com/orbit/. GraphQL reference docs regenerated.
• Run on a Caproni slot: the orbit, group, mutation and type specs pass; Rubocop and markdownlint pass.
• The stable-branch reverts (!265515 (merged), !265516 (merged)) are merged. This MR targets master.
• Branch pipelines still run; [skip ci] only covers MR pipelines.