Core: Remove storyStoreV7 feature flag

[ndelangen]

Relates to #25117

What I did

Step 2 of #24655

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

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 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>

[github-actions]

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🚫	PR is marked with "BREAKING CHANGE" label.
🚫	Please choose only one of these labels: ["BREAKING CHANGE","maintenance"]

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[yannbf]

Please add migration notes that cover all breaking changes, especially when it comes to APIs which could break addons 🙏

These two issues might give a bit of light, but there is definitely much more to cover:
#25367
#25365

[ndelangen]

@yannbf this PR does not remove the actual runtime of storyStoreV7 flag inside the preview code.

This PR removes:

• the ability to set the flag
• the code in the builders to support it

tom's PR that's a follow up for this one, will refactor the preview code.

[shilman]

Looks great. Left a few minor comments!