[Tracking]: Update the Viewport and Background Addon

[valentinpalkovic]

This milestone aims to finalize the schema change for the viewport global, remove feature flags, and improve viewport configuration options in Storybook to align with best practices and Chromatic compatibility.

Items:

1. Remove feature flags committing to the API

• Identify all references to viewport feature flags in the Storybook codebase
  • Remove feature flags from configuration and API references
  • Ensure viewport schema is now the default, without a feature toggle
  • Ensure background schema is now the default, without a feature toggle

2. Update Viewport Global Schema to Support the Legacy Format

• Ensure Storybook 9 supports both viewport: 'iphone13' and viewport: { value: 'iphone13', isRotated: true }

3. Update Background Global Schema to Support the Legacy Format

• Ensure Storybook 9 supports both background: 'red' and background: { value: 'red' }

4. Documentation and Migration Guide

• Update documentation to reflect new viewport behavior
  • Provide migration notes for users transitioning from Storybook 8.x
  • Ensure API documentation clearly explains all viewport configuration options