Drop group from CD models and GraphQL surface

What does this MR do and why?

Align the CD models and GraphQL surface with the organization-only ownership decided in the Rails design proposal and PRD. Group ownership is deferred to a later iteration.

Changes:

• Models: Cd::Application and Cd::Environment no longer read group_id (added ignore_column for 19.3 drop). Removed belongs_to :group and the for_groups / for_namespace scopes.
• GraphQL types: removed the group field and batch loader from CdApplication and CdEnvironment. Granular token boundaries are now instance-only.
• GraphQL mutations: removed the group_path argument, the Mutations::ResolvesGroup include, and the exactly_one_of validation from CdApplicationCreate and CdEnvironmentCreate. organizationId is now required.
• GraphQL resolvers: deleted the group-level Resolvers::Cd::ApplicationsResolver and Resolvers::Cd::EnvironmentsResolver. The organization-level resolvers are unchanged.
• Services and policies: dropped the parent.is_a?(::Group) branch in Cd::Applications::CreateService and Cd::Environments::CreateService; parent is always an organization. Policies now delegate to organization only.
• Group surface: removed has_many :cd_applications / has_many :cd_environments from the Group model, and the matching cdApplications / cdEnvironments fields from the Group GraphQL type.
• Specs, factories, and generated GraphQL docs / introspection schema updated to match.

Notes

• The organization_id sharding key on the cd_* tables was already established in !239900 (merged) (19.1), which added organization_id as the required sharding key across all CD tables while keeping group_id in place as an optional owner.
• Column drops (plus their FKs and unique indexes) follow in 19.3 as a separate post-deploy migration via https://gitlab.com/gitlab-org/gitlab/-/work_items/602934.
• The frontend team has been notified (https://gitlab.com/gitlab-org/gitlab/-/work_items/602765#note_3454308724). The only existing consumer (cd_applications.query.graphql + applications_index.vue) is being migrated to the org-level query in !240759 (merged).

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/602765
• https://gitlab.com/gitlab-org/gitlab/-/work_items/602442
• https://gitlab.com/gitlab-org/gitlab/-/work_items/602444

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