CLI: Add legacyMdx1 & @storybook/mdx1-csf automigration

[ndelangen]

Closes #25433

What I did

apply more migrations listed in #25433

• remove the features. legacyMdx1 field from main.ts
• un-install the @storybook/mdx1-csf package

I do not know how I'd do this:

jsxOptions (addon option from addon-docs) should be removed (though I'm not sure if there's more to it)

Perhaps @shilman or @yannbf should elaborate this task?

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

• Create a sandbox
• force this sandbox to use legacyMdx1
• upgrade this sandbox by running the CLI manually
• expect the legacyMdx1 property to be removed from main.ts features object.
• expect the @storybook/mdx1-csf dependency to get uninstalled.

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>

[valentinpalkovic]

LGTM Code-Wise! Please make sure the feature is tested before merging.