Manager: Add migration note about react 18

[ndelangen]

What I did

I added a migration note informing addon creators about the move to react 18, and the API changes because of it.

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>

[jonniebigodes]

@ndelangen, thank you for taking the time to put together this pull request and add this item to the migration. Appreciate it 🙏 ! I'll check the remainder of the documentation and adjust accordingly in a follow-up pull request. I left a small item for you to look into when you're able. To unblock this, I'm going to approve it so that you can merge it once the feedback is addressed.

[jonniebigodes]

@ndelangen when you have a moment can you adjust this sentence to be:
"The UI added to the manager via addons is now rendered with React 18."

Also, can you capitalize react in the heading and adjust the link above accordingly?