API: Remove stories.json support

[shilman]

Closes N/A

What I did

Remove legacy v6 formats. Note that the manager can still compose v6 storybooks. But v6 storybooks will not be able to compose v8 storybooks (v7 storybooks probably could though).

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

• Build a storybook and validate that stories.json is no longer generated

Documentation

• Update MIGRATION.md

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

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[shilman]

Discussed with @tmeasday and we're OK with this more aggressive approach. @thafryer we'll need to make sure that Chromatic is updated to skip creating a stories.json file when it detects SB8 and above, otherwise we'll create one every time.