Remove views field from the customizable dashboard GraphQL type

What does this MR do and why?

Summary

This change removes the experimental "views" feature from customizable analytics dashboards. The "views" feature would have allowed dashboards to contain multiple switchable sections, each with their own set of panels.

All related code has been cleaned up, including the GraphQL API field, the underlying data model, test fixtures (sample YAML files for dashboards with views), and API tests. The GraphQL schema documentation and introspection file have also been updated to reflect the removal.

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Related to #623802