Core: Remove deprecated createChannel APIs

[yannbf]

Closes #25347

What I did

The createChannel APIs from both @storybook/channel-websocket and @storybook/postmessage are now removed in favor of createBrowserChannel from the @storybook/channels package.

Additionally, the PostmsgTransport type is now removed in favor of PostMessageTransport.

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]

	Fails
🚫	PR is marked with "BREAKING CHANGE" label.

Generated by 🚫 dangerJS against 4583800

[yannbf]

@ndelangen could you please check if the changes are good enough? Also, I don't know whether the migration is really just changing imports. The APIs seem to slightly differ, especially the one in websocket.

@IanVS tagging you too because the code here used to relate to the Vite builder.